ath9k: save a few calls to ath9k_regd_get_eepromRD

Since we already have a stack variable to track the eeprom regd,
we can grab it up front it and save three calls to fetch it again.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Acked-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>