mac80211: preserve more bits when building QoS header

Michael Braun reported that when trying to inject A-MSDUs over
monitor interfaces, the frame doesn't come out right since the
QoS header A-MSDU bit is overwritten.

Rather than adding that bit specifically simply preserve those
bits that we don't set here, since we typically get here with
a zeroed-out QoS header anyway.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>