ahci: Introduce ahci_set_em_messages()

Factor out some ahci_em_messages handling code from ahci_init_one().
We would like to reuse it for non-PCI devices.

Signed-off-by: Anton Vorontsov <avorontsov@ru.mvista.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>