brcm80211: smac: mute transmit on ops_start

Monitor mode functionality (not functional yet) requires transmit to be
muted after ops_start() is called, transmit is unmuted when the first
interface is added.

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>