brcm80211: smac: fixed inconsistency in transmit mute

Transmit was muted in two ways: full mute and a partial mute called
'pre ism cac time' mute. But, this 'pre ism cac time' mute was done at
one place in the code (when tx_mute == false), and overridden later
on in another place in code.

To fix this, the 'pre ism cac time' mute has been replaced by a non
mute.

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>