ahci: replace SATA FIS type magic numbers with constants

SATA 3.0 "10.3.1 FIS Type values" defines the constants used to
differentiate between FIS types.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Message-id: 1415874281-7371-3-git-send-email-stefanha@redhat.com
Signed-off-by: Kevin Wolf <kwolf@redhat.com>