ahci: let users know that Promise PDC42819 support is limited to SATA devices

ahci can drive the Promise PDC42819, but obviously it can only use SATA
disks connected to this controller. The controller can actually support
SAS disks as well, but we only know how to use it in it's AHCI mode.

Add a message to let users know that because ahci is driving their chip
they can only use the SATA disks connected to this controller.

Signed-off-by: Mark Nelson <mdnelson8@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>