mwifiex: maintain station statistic in uap mode

This patch maintain statistic information for the stations associated
to the mwifiex micro AP, include tx/rx bytes/packets, signal strength,
tx bitrate.

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>