[E1 INPUT] explicitly configure the E1 input driver for each line

This introduces a new 'e1_input' config node with a command to be
used like:
  e1_line 0 driver misdn

This allows us to have different input drivers in the future
diff --git a/openbsc/src/e1_config.c b/openbsc/src/e1_config.c
index dd78720..f781b9e 100644
--- a/openbsc/src/e1_config.c
+++ b/openbsc/src/e1_config.c
@@ -51,7 +51,7 @@
 	if (!e1_link->e1_ts)
 		return 0;
 
-	line = e1inp_line_get_create(e1_link->e1_nr);
+	line = e1inp_line_get(e1_link->e1_nr);
 	if (!line)
 		return -ENOMEM;
 
@@ -81,7 +81,7 @@
 		return -EINVAL;
 
 	/* RSL Link */
-	line = e1inp_line_get_create(e1_link->e1_nr);
+	line = e1inp_line_get(e1_link->e1_nr);
 	if (!line)
 		return -ENOMEM;
 	sign_ts = &line->ts[e1_link->e1_ts-1];
@@ -114,7 +114,7 @@
 		return -EINVAL;
 
 	/* OML link */
-	line = e1inp_line_get_create(e1_link->e1_nr);
+	line = e1inp_line_get(e1_link->e1_nr);
 	if (!line)
 		return -ENOMEM;
 	sign_ts = &line->ts[e1_link->e1_ts-1];