ath9k_htc: Fix an endian issue

The stream length/tag fields have to be in little endian
format. Fixing this makes the driver work on big-endian
platforms.

Cc: stable@kernel.org
Tested-by:  <raghunathan.kailasanathan@wipro.com>
Signed-off-by: Sujith Manoharan <Sujith.Manoharan@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>