ms: Store the NACK rate in the MS object

Currently the NACK/unconfirmed ratio is already passed to the
corresponding MS object, but the value is not being stored there.

This commit adds a member and a getter method and include the values
into the output of the 'show ms' command.

Sponsored-by: On-Waves ehf
diff --git a/src/gprs_ms.cpp b/src/gprs_ms.cpp
index fe560e8..b40e1ea 100644
--- a/src/gprs_ms.cpp
+++ b/src/gprs_ms.cpp
@@ -90,7 +90,8 @@
 	m_is_idle(true),
 	m_ref(0),
 	m_list(this),
-	m_delay(0)
+	m_delay(0),
+	m_nack_rate_dl(0)
 {
 	LOGP(DRLCMAC, LOGL_INFO, "Creating MS object, TLLI = 0x%08x\n", tlli);
 
@@ -383,6 +384,8 @@
 	/* TODO: Check for TBF direction */
 	/* TODO: Support different CS values for UL and DL */
 
+	m_nack_rate_dl = error_rate;
+
 	if (error_rate > bts_data->cs_adj_upper_limit) {
 		if (m_current_cs_dl > 1) {
 			m_current_cs_dl -= 1;