libata: cleanup ata_sff_interrupt()

host->ports[i] is never NULL if i < host->n_ports and non-NULL return
from ata_qc_from_tag() guarantees that the returned qc is active.
Drop unnecessary tests.

Superflous () dropped as suggested by Sergei.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>