mwl8787: fix rate index for ofdm rates

Rate indexes for ofdm rates were off by one.  Fixes:

[ 4616.953497] WARNING: at net/mac80211/rx.c:3299 ieee80211_rx+0x77/0x78b [mac80211]()
[ 4616.953506] Modules linked in: mwl8787_sdio hid_generic usbhid hid vfat fat parport_pc ppdev lp parport bnep rfcomm binfmt_misc uinput nfsd nfs_acl nfs fscache lockd sunrpc loop fuse snd_hda_codec_hdmi snd_hda_codec_realtek joydev iTCO_wdt iTCO_vendor_support btusb bluetooth carl9170 ath arc4 coretemp asix kvm_intel libphy iwldvm kvm psmouse mac80211 cdc_ncm evdev cdc_acm serio_raw usbnet mii cdc_wdm iwlwifi pcspkr thinkpad_acpi cfg80211 ac nvram battery snd_hda_intel snd_hda_codec rfkill tpm_tis snd_hwdep i915 snd_pcm snd_page_alloc tpm snd_seq tpm_bios snd_timer lpc_ich snd_seq_device i2c_algo_bit drm_kms_helper i2c_i801 mfd_core snd video wmi acpi_cpufreq mperf drm soundcore button i2c_core processor ext4 mbcache crc16 jbd2 dm_crypt dm_mod sg sd_mod crc_t10dif crc32c_intel ghash_clmulni_intel aesni_intel aes_x86_64 ablk_helper cryptd lrw gf128mul glue_helper ahci libahci libata scsi_mod ehci_pci xhci_hcd ehci_hcd sdhci_pci thermal sdhci thermal_sys usbcore mmc_core usb_common [last unloaded: mwl8787_sdio]
[ 4616.953827] CPU: 1 PID: 14 Comm: ksoftirqd/1 Tainted: G        W    3.10.0+ #78
[ 4616.953834] Hardware name: LENOVO 3443CTO/3443CTO, BIOS G6ET23WW (1.02 ) 08/14/2012
[ 4616.953839]  0000000000000002 ffff88011979bc68 ffffffff813b827e ffff88011979bca0
[ 4616.953854]  ffffffff81032dd4 0000000000000000 ffff88008d894a60 ffff88008100f0c0
[ 4616.953867]  ffff88008d894cf0 0000000000000000 ffff88011979bcb0 ffffffff81032e06
[ 4616.953881] Call Trace:
[ 4616.953900]  [<ffffffff813b827e>] dump_stack+0x19/0x1b
[ 4616.953912]  [<ffffffff81032dd4>] warn_slowpath_common+0x65/0x7d
[ 4616.953922]  [<ffffffff81032e06>] warn_slowpath_null+0x1a/0x1c
[ 4616.953979]  [<ffffffffa055b6b6>] ieee80211_rx+0x77/0x78b [mac80211]
[ 4616.954037]  [<ffffffff813bc5e1>] ? _raw_spin_unlock_irqrestore+0x3f/0x6e
[ 4616.954055]  [<ffffffff810812f2>] ? trace_hardirqs_on_caller+0x16/0x1ad
[ 4616.954080]  [<ffffffff813bc602>] ? _raw_spin_unlock_irqrestore+0x60/0x6e
[ 4616.954130]  [<ffffffffa053c8cf>] ieee80211_tasklet_handler+0x3f/0xa6 [mac80211]
[ 4616.954164]  [<ffffffff8103aae0>] tasklet_action+0x86/0xdd
[ 4616.954189]  [<ffffffff8103a626>] __do_softirq+0x137/0x2a1
[ 4616.954226]  [<ffffffff8103a7b3>] run_ksoftirqd+0x23/0x53
[ 4616.954249]  [<ffffffff8105b5d9>] smpboot_thread_fn+0x242/0x260
[ 4616.954272]  [<ffffffff8105b397>] ? test_ti_thread_flag.constprop.2+0x11/0x11
[ 4616.954292]  [<ffffffff8105422d>] kthread+0xb1/0xb9
[ 4616.954324]  [<ffffffff8105417c>] ? __kthread_parkme+0x65/0x65
[ 4616.954358]  [<ffffffff813c2edc>] ret_from_fork+0x7c/0xb0
[ 4616.954382]  [<ffffffff8105417c>] ? __kthread_parkme+0x65/0x65
[ 4616.954399] ---[ end trace ff659b733f1a37c4 ]---

Signed-off-by: Bob Copeland <me@bobcopeland.com>