tbf: Implement enable_egprs() once

There's no real need for having different copies of this method in each
children. Furthermore, having the method implemented in the base class
made me shoot my foot while trying to move this to the tbf constructor
(see next commit), so let's simplify this and avoid other people
following into the same issue.

enable_egprs() in tbf.h is moved to be public since it needed (as it was
for the duplicated children mehtods with same name), but anyway it will
be moved to private in next commit.

Change-Id: Id7de060318201a42e51f277f898463f4b9a84eba
diff --git a/src/tbf.h b/src/tbf.h
index bf51a8d..ac6c63c 100644
--- a/src/tbf.h
+++ b/src/tbf.h
@@ -263,6 +263,7 @@
 	/* EGPRS */
 	bool is_egprs_enabled() const;
 	void disable_egprs();
+	void enable_egprs();
 
 	/* attempt to make things a bit more fair */
 	void rotate_in_list();
@@ -322,7 +323,6 @@
 
 protected:
 	gprs_rlcmac_bts *bts_data() const;
-	void enable_egprs();
 	void merge_and_clear_ms(GprsMs *old_ms);
 
 	gprs_llc_queue *llc_queue();
@@ -581,6 +581,7 @@
 inline void gprs_rlcmac_tbf::enable_egprs()
 {
 	m_egprs_enabled = true;
+	window()->set_sns(RLC_EGPRS_SNS);
 }
 
 inline void gprs_rlcmac_tbf::disable_egprs()