LAPDM: Use correct offset to short header on recevied frame

The offset of the short header on main DCCH is 0, not 2.

Change-Id: I8345776768fdf2a700b2ca1d117f9ef4b15777cc
---
M src/gsm/lapdm.c
1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/src/gsm/lapdm.c b/src/gsm/lapdm.c
index 9f795c6..86dc242 100644
--- a/src/gsm/lapdm.c
+++ b/src/gsm/lapdm.c
@@ -754,7 +754,7 @@
 			msg->l2h += 2;
 		} else {
 			/* A Short L3 header has both bits == 0. */
-			if (LAPDm_ADDR_SHORT_L2(msg->l2h[2]) == 0) {
+			if (LAPDm_ADDR_SHORT_L2(msg->l2h[0]) == 0) {
 				mctx.lapdm_fmt = LAPDm_FMT_Bter;
 				n201 = N201_Bter_SDCCH;
 				sapi = 0;

