[SCSI] sg: remove unnecessary check

coverity spotted this (cid #758).  All callers dereference sfp, so we dont
need this check.  In addition to this, we dereference it earlier in the
function.

Signed-off-by: Eric Sesterhenn <snakebyte@gmx.de>
Acked-by: Douglas Gilbert <dougg@torque.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: James Bottomley <James.Bottomley@SteelEye.com>