TBF: add N3101 counter

Properly reset the counter when receiving valid RLCMAC block and update
it when no data is received as per 3GPP TS 44.060 ยง8.1.1.1

Change-Id: I2f79c6153dc4073c9d293b2824979e6381576682
Fixes: OS#2407
diff --git a/src/tbf.h b/src/tbf.h
index 06c9f60..4489695 100644
--- a/src/tbf.h
+++ b/src/tbf.h
@@ -296,6 +296,8 @@
 	/* store the BTS this TBF belongs to */
 	BTS *bts;
 
+	uint8_t m_n3101; /* N3101 counter */
+
 	/*
 	 * private fields. We can't make it private as it is breaking the
 	 * llist macros.