ath9k_htc: Process command data properly

When handling the REGIN callback, processing
the incoming data first should be the preferred
mode of operation. Allocation of a new SKB may fail,
in which case, the URB will not be resubmitted.

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