scsi: mptsas: fix the wrong reading size in fetch request

When fetching request, it should read sizeof(*hdr), not the
pointer hdr.

Signed-off-by: Li Qiang <liqiang6-s@360.cn>
Message-Id: <1489488980-130668-1-git-send-email-liqiang6-s@360.cn>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>