ath9k: fix moredata flag endianness in cabq tx

While compiling ath9k with some extra flags I've
found that:

 ath9k/xmit.c +2473 ## 16: warning: restricted __le16 degrades to integer
 ath9k/xmit.c +2474 ## 36: warning: invalid assignment: &=
 ath9k/xmit.c +2474 ## 36:    left side has type restricted __le16
 ath9k/xmit.c +2474 ## 36:    right side has type int

There's no way for frame ftype/stype to be
mistreated as the offending 'moredata' flag when
considering cab queue.

This could've however theoretically led sometimes
to increased power consumption on connected
stations as they would keep their Rx active
waiting for frames that would never come.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>