pata_serverworks: cleanup cable detection

Merge identical cable routines for Dell and Sun systems into
common oem_cable() one.

Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <bzolnier@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>