ath5k: run NF calibration only every 60 seconds

Since NF calibration interferes with TX and RX and also has been the cause of
other problems (when it's run concurrently with ath5k_reset) we want to run it
less often - every 60 seconds for now.

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