Wireless / ath5k: Simplify suspend and resume callbacks

Simplify the suspend and resume callbacks of ath5k by converting the
driver to struct dev_pm_ops and allowing the PCI PM core to do the
PCI-specific suspend/resume handling.

Signed-off-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: John W. Linville <linville@tuxdriver.com>