mac80211: Use only precedence level of DSCP field for frame classification

Bit 4-5 of DSCP should not be considered by classify_d1. The
802.11 QoS Priority field is only depending on the precedence level.

Signed-off-by: Daniel Wagner <wagi@monom.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>