pdch: Instead of passing bts, trx, ts use the pdch

All dispatching will go through the PDCH. This will clean a lot
of the look-ups inside the gprs_rlcmac_data.c and continue with
adding structure to the pcu code.
diff --git a/src/bts.h b/src/bts.h
index a8b6223..4b9ede4 100644
--- a/src/bts.h
+++ b/src/bts.h
@@ -53,6 +53,9 @@
 
 	void enable();
 	void disable();
+
+	/* dispatching of messages */
+	int rcv_block(uint8_t *data, uint8_t len, uint32_t fn, int8_t rssi);
 #endif
 
 	uint8_t m_is_enabled; /* TS is enabled */