drivers/net/wireless/rt2x00/rt2x00dev.c: remove null test before kfree

Fix checkpatch warning:
WARNING: kfree(NULL) is safe this check is probably not required

Cc: Ivo van Doorn <IvDoorn@gmail.com>
Cc: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: John W. Linville <linville@tuxdriver.com>