LAPDm: Add an extra queue for UI frames

The extra queue is used to transmit the UI frame only when there is no
frame in the regular TX queue. This allows to give LAPD frames prioity
over UI frame.

Related: OS#4074
Change-Id: I00c8ee73be8b7c564a4dee3fca3e893484f567da
diff --git a/TODO-RELEASE b/TODO-RELEASE
index 226450b..fa7bc57 100644
--- a/TODO-RELEASE
+++ b/TODO-RELEASE
@@ -11,3 +11,4 @@
 core      ADD       gsmtap_inst_fd2() core, DEPRECATE gsmtap_inst_fd()
 isdn		ABI change		add states and flags for external T200 handling
 gsm		ABI change		add T200 timer states to lapdm_datalink
+gsm		ABI change		add UI queue to struct lapdm_datalink