mwifiex: toggle carrier state in start_ap/stop_ap.

In uap mode the carrier is not enabled until after the first STA joins.
The carrier triggers the bridge to start its state machine, and if STP
is enabled, it takes 4 seconds as it transitions from disabled to
forwarding. During this time the bridge drops all traffic, and the EAPOL
handshake times out after 3 seconds, preventing stations from joining.

Follow the logic used in mac80211 and start the carrier in start_ap
and disable it in stop_ap. This has a nice benefit of allowing the
first station connection time to be reduced by up to 75% when STP is
in use.

Signed-off-by: Martin Faltesek <mfaltesek@google.com>
Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>