mwl8k: prevent module unload hang

We need to unregister our ieee80211_hw before resetting the chip, as
the former causes firmware commands to be issued which will time out
once the chip has been reset.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Acked-by: Nicolas Pitre <nico@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>