lapdm: Fix back-pointer from lapdm_entity to lapdm_channel

It seems that during all those years it has never been noted that
the back-pointer from the lapdm_entity to the lapdm_channel was
never initialized.  Let's fix that.

Change-Id: Iaca66cd6a2c9f315561e365b51163927868fc346
diff --git a/src/gsm/lapdm.c b/src/gsm/lapdm.c
index 6a16012..ae21ccd 100644
--- a/src/gsm/lapdm.c
+++ b/src/gsm/lapdm.c
@@ -169,8 +169,10 @@
 void lapdm_channel_init(struct lapdm_channel *lc, enum lapdm_mode mode)
 {
 	lapdm_entity_init(&lc->lapdm_acch, mode, 2);
+	lc->lapdm_acch.lapdm_ch = lc;
 	/* FIXME: this depends on chan type */
 	lapdm_entity_init(&lc->lapdm_dcch, mode, 1);
+	lc->lapdm_dcch.lapdm_ch = lc;
 }
 
 /*! flush and release all resoures in LAPDm entity */