brcm80211: smac: modified Mac80211 callback interface

Upon ops_start(), a Mac80211 driver should enable receive functionality to
support monitor mode. Also, upon ops_stop(), it should disable rx.

Driver did not follow this rule so code has been changed.

Reported-by: Johannes Berg <johannes@sipsolutions.net>
Reviewed-by: Alwin Beukers <alwin@broadcom.com>
Reviewed-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: Roland Vossen <rvossen@broadcom.com>
Signed-off-by: Arend van Spriel <arend@broadcom.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>