rt2x00: Fix compile warning when debug disabled

CC [M]  drivers/net/wireless/rt2x00/rt2800lib.o
drivers/net/wireless/rt2x00/rt2800lib.c: In function 'rt2800_ampdu_action':
drivers/net/wireless/rt2x00/rt2800lib.c:2821: warning: unused variable 'rt2x00dev'

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Acked-by: Helmut Schaa <helmut.schaa@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>