ath9k_htc: Move endpoint header parsing to TX tasklet

There is no need to do endpoint header removal in the ISR.
Also, this is needed when TX slot management is added later on.
Use a helper function to strip the driver header.

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