Split csn1.c into common, enc and dec files

The CSN1 encoder/decoder code is already lengthy and complex enough,
there's no need to keep it in the same file, specially because when
debugging, only is interested in one of the 2 functions, and they both
look really similar (long spaghetti switches).

Change-Id: I7d1b1f7e6d7f89b052b3fd73a960419bb2673020
diff --git a/debian/copyright b/debian/copyright
index 99ea408..4e9347e 100644
--- a/debian/copyright
+++ b/debian/copyright
@@ -81,7 +81,9 @@
 License:   GPL-2.0+
 
 Files:     src/csn1.h
-           src/csn1.cpp
+           src/csn1.c
+           src/csn1_dec.c
+           src/csn1_enc.c
            src/gsm_rlcmac.cpp
            src/gsm_rlcmac.h
 Copyright: 2011 Vincent Helfre