ath9k_hw: Offload USB eeprom reading to target

For USB devices, reading the EEPROM data can be offloaded
to the target. Use multiple register reads to take advantage
of this feature to reduce initialization time.

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