sata_promise: disable hotplug on 1st gen chips

1st generation Promise SATA chips are prone to generating spurious
hotplug events which can disrupt normal operation. This has been
observed on 20376 and 20378 chips. This patch thus disables hotplug
support on 1st gen chips while leaving it enabled for 2nd gen chips.

The pdc_sata_hotplug_offset() function becomes redundant so it is
removed.

Tested on 1st gen 20376 and 20378 mainboard chips and on a 2nd gen
SATA300 PCI card.

Signed-off-by: Mikael Pettersson <mikpe@it.uu.se>
Tested-by: Kurt Roeckx <kurt@roeckx.be>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>