rtlwifi: Implement a common rtl_phy_scan_operation_backup() routine

Several of the drivers supported by rtlwifi individually implement the same
routine that supports scans. As a first step, create this routine in
driver rtlwifi.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>