wl1251: cleanup scanning code

The current scanning code wasn't following the preferred style. Move code
related to scan and trigger scan to commans to wl1251_cmd.c. Because
there's now less code in wl1251_hw_scan(), the function can be now
merged with wl1251_op_hw_scan().

Signed-off-by: Kalle Valo <kalle.valo@nokia.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>