rt2x00: Filter ACK_CTS based on FIF_CONTROL

The ACK_CTS frame is a control frame, this means
dropping the frame depends on the FIF_CONTROL flag
for filtering.

This also fixes an obvious typo in register definition.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>