ath9k_htc: Fix register read through bulk pipe

To optimize register read/write operations, the HTC firmwares were patched
to change EP3 and EP4 pipe types from Interrupt to Bulk. So register writes
are submitted as bulk urbs, but register reads are not. Also changing the
register read endpoint pipe as bulk type when URBs are filled improves the
register reads considerably which results in reduced scan time and CPU
utilization.

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