tbf: First round of removing llc handling from the rlcmac_data

The code in gprs_rlcmac_data should ask the TBF for help in packing
the frames but it really shouldn't poke in the internals of the
tbf structure. This is very bad capsuling and has plenty of copy
and paste.

At the same thime this will be the most dangerous refactoring of
the code base.
diff --git a/src/tbf.h b/src/tbf.h
index addf651..0441096 100644
--- a/src/tbf.h
+++ b/src/tbf.h
@@ -22,6 +22,8 @@
 
 #include <stdint.h>
 
+struct bssgp_bvc_ctx;
+
 /*
  * TBF instance
  */
@@ -90,6 +92,10 @@
 	bool state_is_not(enum gprs_rlcmac_tbf_state rhs) const;
 	void set_state(enum gprs_rlcmac_tbf_state new_state);
 
+	/* TODO: add the gettimeofday as parameter */
+	struct msgb *llc_dequeue(bssgp_bvc_ctx *bctx);
+	void update_llc_frame(struct msgb *msg);
+
 	int rlcmac_diag();
 
 	struct llist_head list;