mwl8787: set correct MCS idxes for STA

The mwl8787 firmware has a rate idx for each MCS 0-7,
which are then repeated in the next 8 bits for each MCS0-7
@ 40MHz. Allows the firmware to realize a peer is
HT40-capable.

Signed-off-by: Thomas Pedersen <thomas@cozybit.com>
mwl8787: set 11n capabilities along with channel

Needed to tell the fw we want HT40 / SGI. Might need
something similar for per-peer later.

Signed-off-by: Thomas Pedersen <thomas@cozybit.com>