rt2x00: Fix SM PS check

Fix a check for dynamic SM PS mode in the STAs HT caps. Since a
value of 3 means "SM PS disabled" the previous check assumed in
that case that "dynamic SM PS" was enabled and as such prefixed
every MCS>7 frame with a unnecessary RTS/CTS exchange. Also,
the bit shift was done in the wrong direction.

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>