[PATCH] libata: trivial updates to ata_sg_init_one()

There's no need to memset &qc->sgent manually, sg_init_one() clears
sgent inside it.  Also, kill not-so-necessary sg local variable.

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