wl12xx: flush packets before stopping dev role

During sta disconnection, a deauth packet is being queued to
the dev role queue. However, the dev role is being stopped
before the packet was sent.

Flush the tx queue before stopping the dev role.

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <coelho@ti.com>