ath5k: move ath5k_hw_calibration_poll to base.c

It's not a phy related funtion; It has more to do with the interrupt handler
and tasklet scheduling, so it belongs to base.c.

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