[input] make mi_e1_line_update() a driver specific callback

This helps us to further disentangle the misdn driver and e1_input
in preparation of DAHDI support.
diff --git a/openbsc/src/e1_input.c b/openbsc/src/e1_input.c
index a17ae41..45231c3 100644
--- a/openbsc/src/e1_input.c
+++ b/openbsc/src/e1_input.c
@@ -542,7 +542,10 @@
 
 int e1inp_line_update(struct e1inp_line *line)
 {
-	return mi_e1_line_update(line);
+	if (line->driver && line->driver->line_update)
+		return line->driver->line_update(line);
+	else
+		return 0;
 }
 
 static int e1i_sig_cb(unsigned int subsys, unsigned int signal,