This patch performs two simple cleanups of sata_promise.

* Remove board_20771 and map device id 0x3577 to board_2057x.
  After the recent corrections for SATAII chips, board_20771 and
  board_2057x were equivalent in the driver.

* Remove hp->hotplug_offset and use hp->flags & PDC_FLAG_GEN_II
  to compute hotplug_offset in pdc_host_init(). hp->hotplug_offset
  was used to distinguish 1st and 2nd generation chips in one
  particular case, but now we have that information in a more
  general form in hp->flags, so hp->hotplug_offset is redundant.

Changes since previous submission: rebased on libata-dev #upstream,
cleaned up hotplug_offset computation based on Tejun's comments,
expanded hotplug_offset removal rationale.

This patch does not depend on the pending new EH conversion patch.
Signed-off-by: Mikael Pettersson <mikpe@it.uu.se>
Signed-off-by: Jeff Garzik <jeff@garzik.org>