mac80211: preserve EOSP in QoS header

Janusz reported that the EOSP bit in mac80211 was
getting cleared all the time. I had not found this
since I tested uAPSD with a device that always set
the bit itself. Preserve the bit when building the
QoS header.

Reported-by: Janusz Dziedzic <janusz.dziedzic@gmail.com>
Tested-by: Janusz Dziedzic <janusz.dziedzic@gmail.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>