tbf: Use type bool for upgrade_to_multislot

Change-Id: I644d91b6230a90cc72e83443c11d24b8d0a2dcac
diff --git a/src/tbf.h b/src/tbf.h
index 96a5788..4218cf5 100644
--- a/src/tbf.h
+++ b/src/tbf.h
@@ -265,7 +265,7 @@
 	} meas;
 
 	/* Can/should we upgrade this tbf to use multiple slots? */
-	uint8_t upgrade_to_multislot;
+	bool upgrade_to_multislot;
 
 	/* store the BTS this TBF belongs to */
 	struct gprs_rlcmac_bts *bts;