mwifiex: do not update MCS set from hostapd

We should not copy the MCS set from hostapd RX-STBC. We
have to just use the MCS set supported by the hardware.
This fixes an issue, where mwifiex is advertising wrong
MCS sets in beacons.

Fixes: 474a41e9 ("mwifiex: update MCS set as per RX-STBC bit from hostapd")
Signed-off-by: Ganapathi Bhat <gbhat@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>