pata_sis: extract a sis_port_base() method

This is similar to the existing sis_old_port_base() method. We do this
same calculation and logic in multiple places (with one more to come in
a future patch), so extracting it into a method makes sense.

Reviewed-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Signed-off-by: Dan McGee <dpmcgee@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>