ath9k: Define bus agnostic bluetooth coex prep helper

We disable ASPM when enabling bluetooth coexistance. Disabling
ASPM is a bus specific operation. In the future other buses may
support bluetooth coexistance, an example is USB. To this end
move the current routine which disables ASPM into pci.c, and declare
it the PCI bt_coex_prep() helper. Additionally, since ASPM is
a PCI-Express primitive ensure we don't ever try to muck with ASPM
registers on non PCI-express devices.

This also cleans up hw.c to not include bus specific headers or
utilities.

Cc: Vasanthakumar Thiagarajan <vasanth@atheros.com>
Cc: Stephen Chen <stephen.chen@atheros.com>
Cc: Zhifeng Cai <zhifeng.cai@atheros.com>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>