abis: add init function for emulated ipaccess/HSL emulated E1 input

This patch is a cleanup. It adds an init function for the ipaccess
and HSL emulated E1 input drivers.
diff --git a/openbsc/src/libabis/input/ipaccess.c b/openbsc/src/libabis/input/ipaccess.c
index 8080129..1427118 100644
--- a/openbsc/src/libabis/input/ipaccess.c
+++ b/openbsc/src/libabis/input/ipaccess.c
@@ -812,12 +812,6 @@
 {
 	int ret;
 
-	/* register the driver with the core */
-	/* FIXME: do this in the plugin initializer function */
-	ret = e1inp_driver_register(&ipaccess_driver);
-	if (ret)
-		return ret;
-
 	e1h = talloc_zero(tall_bsc_ctx, struct ia_e1_handle);
 	if (!e1h)
 		return -ENOMEM;
@@ -838,3 +832,8 @@
 
 	return ret;
 }
+
+void e1inp_ipaccess_init(void)
+{
+	e1inp_driver_register(&ipaccess_driver);
+}