mwifiex: correct antenna number with high bits reserved

High bits of antenna number are reserved in hardware spec,
using low 4 bits represent supported antenna.

Signed-off-by: Cathy Luo <cluo@marvell.com>
Signed-off-by: Xinming Hu <huxm@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>