mwifiex: change memset to simple assignment for ht_cap.mcs.rx_mask

WARNING: single byte memset is suspicious.
         Swapped 2nd/3rd argument?

This code happens to work because rx_mcs is the first variable
in mcs structure. We should use 'mcs.rx_mcs' here anyway.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Maithili Hinge <maithili@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>