mwifiex: fix potential out-of-boundary access to ibss rate table

commit 5f0fabf84d7b52f979dcbafa3d3c530c60d9a92c upstream.

smatch found this error:

CHECK   drivers/net/wireless/mwifiex/join.c
  drivers/net/wireless/mwifiex/join.c:1121
  mwifiex_cmd_802_11_ad_hoc_join()
  error: testing array offset 'i' after use.

Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>