ath9k_htc: Validate TX Endpoint ID

Check for the endpoint IDs when processing
TX completions and drop the unsupported EPIDs.

We can add other endpoints (UAPSD,..) when support
is added.

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