mwl8k: fix configure_filter() memory leak on error

If there was an error acquiring the firmware lock in
mwl8k_configure_filter(), we would end up leaking the multicast
command packet prepared by mwl8k_prepare_multicast().

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