mwl8k: revert unnecessary modification of tx descriptor

This reverts change 783391c443728febc669e40597193308460e7b4f.
The stabilized AP v1 firmware uses the same tx descriptor as
the STA firmware.

Signed-off-by: Brian Cavagnolo <brian@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>