rt2x00: fix oops in rt2x00lib_txdone with rt61pci

Fix a typo introduced in "rt2x00: Add helper function for reporting tx
status" that results in an oops in rt2x00lib_txdone.

Reported-by: Pavel Roskin <proski@gnu.org>
Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>