llc: Convert to C

There's no real reason (other than historical) why code in llc should be
kept as c++ code. Let's rewrite it as C so that it can be included by
existing C code without having to add C->C++ shim, ifdefs all around,
etc.
This simplifies code and easies modification/improvement of the related
objects.

Change-Id: I250680ba581167d7398b2f734769c756cbb61c48
diff --git a/debian/copyright b/debian/copyright
index 4e9347e..4a2b5dd 100644
--- a/debian/copyright
+++ b/debian/copyright
@@ -42,7 +42,7 @@
            src/tbf_dl.cpp
            src/sba.cpp
            src/sba.h
-           src/llc.cpp
+           src/llc.c
            src/encoding.cpp
            src/encoding.h
            src/gprs_rlcmac.cpp
@@ -50,6 +50,7 @@
 Copyright: 2012 Ivan Klyuchnikov
            2012 Andreas Eversberg <jolly@eversberg.eu>
            2013 by Holger Hans Peter Freyther
+           2022 by by Sysmocom s.f.m.c. GmbH
 License:   GPL-2.0+
 
 Files:     src/gprs_rlcmac.h