ath9k_htc: Fix HTC layer memleak

Messages that are generated by the HTC layer
don't have any TX callback endpoints assigned to them.
Consequently, the allocated SKBs are never freed.

Fix this issue by handling this case in the HTC layer
itself.

Signed-off-by: Sujith <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>