sata_sil: enable 32-bit PIO

32-bit PIO seems to work fine on sata_sil hardware (tested on SiI3114) and is
listed as OK in the Silicon Image datasheets. Enable it.

Signed-off-by: Robert Hancock <hancockrwd@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>