tbf: Move the code to close all timeslots into the tbf class

Move the code out of the sysmocom_socket (as this is only required
to talk to our hardware) and into the TBF class that actually knows
what a TBF is.
diff --git a/src/tbf.cpp b/src/tbf.cpp
index 79f1ea8..8aff463 100644
--- a/src/tbf.cpp
+++ b/src/tbf.cpp
@@ -237,3 +237,17 @@
 
 	return tbf;
 }
+
+void gprs_rlcmac_tbf::free_all(struct gprs_rlcmac_trx *trx)
+{
+	for (uint8_t tfi = 0; tfi < 32; tfi++) {
+		struct gprs_rlcmac_tbf *tbf;
+
+		tbf = trx->ul_tbf[tfi];
+		if (tbf)
+			tbf_free(tbf);
+		tbf = trx->dl_tbf[tfi];
+		if (tbf)
+			tbf_free(tbf);
+	}
+}