ath5k: Disable interrupts in ath5k_hw_get_tsf64

The code in ath5k_hw_get_tsf64() is time critical and will return wrong results
if we get interrupted, so disable local interrupts.

Signed-off-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>