ath9k_htc: Fix CPU usage issue during scan period

The CPU consumption during the scan period is high, since
the register write go over Interrupt endpoint. On downloading
the firmware to the target, the USB descriptors are
'patched' to change the type of the endpoints from Interrupt
to Bulk.

With this fix, the CPU usage during a scan run comes down to
acceptable levels.

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