ath9k_hw: Fix usec to hw clock conversion in 5Ghz for ar9003

Fast clock operation (44Mhz) is enabled for 5Ghz in ar9003, so
take care of the conversion from usec to hw clock.

Signed-off-by: Vasanthakumar Thiagarajan <vasanth@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>