mwl8k: do not free unrequested irq

When the mwl8k driver attempts and fails to switch from sta to ap
firmware (or vice-versa) in the mwl8k_add_interface routine, the
mwl8k_stop routine will be called. This routine must not attempt
to free the irq if it was not requested.

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