mwifiex: add sta_list firmware command

This patch add sta_list firmware command, which can be used
to get power status and rssi for the stations associated to
mwifiex micro AP.

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