zd1211rw: Fix data padding for QoS

This patch fixes a data alignment issue in the zd1211rw driver.
The IEEE80211_STYPE_QOS_DATA bit should be used as a bitwise test
to test for the presence of the 2 byte QoS control field.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>