mwifiex: implement remain_on_channel and cancel_remain_on_channel

Add a new command to implement remain_on_channel and
cancel_remain_on_channel.

Signed-off-by: Stone Piao <piaoyun@marvell.com>
Signed-off-by: Kiran Divekar <dkiran@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>