mwl8k: Implement sw_scan start/stop cbs

Implement sw_scan start and stop callbacks.
Reset BBP registers for channel, RX busy and
average noise stats which clear on read.

Signed-off-by: Yogesh Ashok Powar <yogeshp@marvell.com>
Signed-off-by: Nishant Sarmukadam <nishants@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>