target/iscsi: Fix mutual CHAP auth on big-endian arches

commit 7ac9ad11b2a5cf77a92b58ee6b672ad2fa155eb1 upstream.

See https://bugzilla.redhat.com/show_bug.cgi?id=916290

Used a temp var since we take its address in sg_init_one.

Signed-off-by: Andy Grover <agrover@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>