ath9k: Convert to new PCI PM framework

The ath9k driver uses the legacy PCI power management (suspend
and resume) callbacks that apparently cause intermittent problems
to happen (the adapter sometimes doesn't resume correctly on my
Acer Ferrari One).  Make it use the new PCI PM and let the PCI core
code handle the PCI-specific details of power transitions.

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