rt2x00: fix a possible NULL pointer dereference

The 'rt2x00lib_probe_dev' function tries to
allocate the workqueue. If the allocation
fails, 'rt2x00_lib_remove_dev' is called on
the error path. Because 'rt2x00dev->workqueue'
is NULL in this case, the 'destroy_workqueue'
call will cause a NULL pointer dereference.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>