[misdn] Add option to automatically release layer2 on exit

Patch by Andreas Eversberg to automatically release
layer2 on exit of the application. Made the naming of
the variables consistent (only release_l2 and not both
release_l2 and l2_release).
diff --git a/include/openbsc/misdn.h b/include/openbsc/misdn.h
index b6bd391..d3631e7 100644
--- a/include/openbsc/misdn.h
+++ b/include/openbsc/misdn.h
@@ -22,7 +22,7 @@
 
 #include "e1_input.h"
 
-int mi_setup(int cardnr,  struct e1inp_line *line);
+int mi_setup(int cardnr,  struct e1inp_line *line, int release_l2);
 int _abis_nm_sendmsg(struct msgb *msg);
 
 #endif