ath9k_hw: add pcieSerDesWrite to disable SERDES ASPM tweaks

This can be useful during testing of new ASPM tweaks which often
have to be done through the PCI Serializer-Deserializer (SERDES).

Cc: Aeolus Yang <aeolus.yang@atheros.com>
Cc: Madhan Jaganathan <madhan.jaganathan@atheros.com>
Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>