ath9k_hw: fix and clean up PHY activation delay

The delay calculation is the same for all chips, however some parts of the
code missed the extra delay factor for half/quarter.
Clean up the code and move the delay calculation to a common place.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>