ms: Add back pointer to BTS

Since more functionality will be moved to the GprsMs class, a pointer
to the current BTS object is added to allow access to configuration
data and other methods.

Sponsored-by: On-Waves ehf
diff --git a/src/bts.cpp b/src/bts.cpp
index ae0a1ca..608bd0e 100644
--- a/src/bts.cpp
+++ b/src/bts.cpp
@@ -100,6 +100,7 @@
 	: m_cur_fn(0)
 	, m_pollController(*this)
 	, m_sba(*this)
+	, m_ms_store(this)
 {
 	memset(&m_bts, 0, sizeof(m_bts));
 	INIT_LLIST_HEAD(&m_bts.ul_tbfs);