drivers/net/wireless/mwl8k.c: avoid use-after-free

Do not dereference p->station_id after kfree(cmd) because p
points into the cmd data structure.

Signed-off-by: Nickolai Zeldovich <nickolai@csail.mit.edu>
Signed-off-by: John W. Linville <linville@tuxdriver.com>