TBF: make network counters internal

* store N310* counters in shared array similar to corresponding timers
* add functions to increment/reset counters

This avoids direct access to TBF counters from PDCH.

Change-Id: I8ffff9c7186f74bde7e6ac5f6e98f0b3e4c35274
Related: OS#1539
diff --git a/src/tbf_ul.cpp b/src/tbf_ul.cpp
index a4d3499..02f4ddb 100644
--- a/src/tbf_ul.cpp
+++ b/src/tbf_ul.cpp
@@ -333,7 +333,7 @@
 			LOGPTBFUL(this, LOGL_DEBUG, "Finished with UL TBF\n");
 			TBF_SET_STATE(this, GPRS_RLCMAC_FINISHED);
 			/* Reset N3103 counter. */
-			this->m_n3103 = 0;
+			this->n_reset(N3103);
 		}
 	}