ath9k_htc: Introduce new HTC API

A new routine that takes an endpoint explicitly is
introduced. The normal htc_send() now retrieves the endpoint
from the packet's private data. This would be useful
in TX completion when the endpoint ID would be required.
While at it, use a helper function to map the queue to endpoint.

Data/mgmt/beacon packets use htc_send(), while WMI comamnds
pass the endpoint to HTC.

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