[PATCH] ahci: preserve PORTS_IMPL over host resets

Instead of writing 0xf blindly, preserve the content of write-once
PORTS_IMPL register over host resets.

This patch is taken from Jeff Garzik's AHCI init update patch.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>