[gprs] Ensure msgb->l3h points to Layer3 (04.08)

In the old code
	l3h = BSSGP, l4h = LLC, cb[gmmh] = 04.08
Now, this has been changed to
	cb[bssgph] = BSSGP, cb[llch] = LLC, l3h = 04.08

This way, GSM general 04.08 and GPRS 04.08 code can expect a
GSM 04.08 header at msgb->l3h
diff --git a/openbsc/src/gprs_ns.c b/openbsc/src/gprs_ns.c
index 7f94b97..470ccb0 100644
--- a/openbsc/src/gprs_ns.c
+++ b/openbsc/src/gprs_ns.c
@@ -266,7 +266,7 @@
 
 	/* spare octet in data[0] */
 	bvci = nsh->data[1] << 8 | nsh->data[2];
-	msg->l3h = &nsh->data[3];
+	msgb_bssgph(msg) = &nsh->data[3];
 
 	/* call upper layer (BSSGP) */
 	return nsvc->nsi->cb(GPRS_NS_EVT_UNIT_DATA, nsvc, msg, bvci);