ath9k_hw: bail out early on ath9k_hw_init_rf()

We a huge branch for old hardware and nothing for newer
hardware. Instead of doing this just bail out early for
newer hardware.

This patch has no functional changes.

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