ath5k: fix SWI calibration interrupt storm

The calibration period is now invoked by triggering a software
interrupt from within the ISR by ath5k_hw_calibration_poll()
instead of via a timer.

However, the calibration interval isn't initialized before
interrupts are enabled, so we can have a situation where an
interrupt occurs before the interval is assigned, so the
interval is actually negative.  As a result, the ISR will
arm a software interrupt to schedule the tasklet, and then
rearm it when the SWI is processed, and so on, leading to a
softlockup at modprobe time.

Move the initialization order around so the calibration interval
is set before interrupts are active.  Another possible fix
is to schedule the tasklet directly from the poll routine,
but I think there are additional plans for the SWI.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Cc: stable@kernel.org
Signed-off-by: John W. Linville <linville@tuxdriver.com>