ath5k: don't use PCI ID to find the chip revision

AR5K_SREV is available even if the chip has been put to sleep.  Relying
on the chip register allows binding non-standard PCI IDs by

echo VENDOR_ID PRODUCT_ID >/sys/bus/pci/drivers/ath5k/new_id

without having to specify the driver data as well.

Signed-off-by: Pavel Roskin <proski@gnu.org>
Acked-by: Bob Copeland <me@bobcopeland.com>
Acked-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>