rt2x00: Optimize unmapping of skbs

Since no skb will be mapped for RX and TX at the same time we can
simply shortcut the check for SKBDESC_DMA_MAPPED_TX.

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