mwifiex: correction in Rx STBC field of htcapinfo

Currently Rx STBC in assoc request frame is advertised as 3. It should
be 2, as our chipsets support two spatial streams.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>