e1_input: minor API changes to adapt it to openbsc

While working on the openbsc over libosmo-abis port, I noticed
several API changes that we need to perform for better adaptation.
diff --git a/src/input/dahdi.c b/src/input/dahdi.c
index bf73f97..20c2a09 100644
--- a/src/input/dahdi.c
+++ b/src/input/dahdi.c
@@ -122,8 +122,8 @@
 
 	idata = lapd_receive(e1i_ts->driver.dahdi.lapd, msg->data, msg->len, &ilen, &prim);
 	if (!idata && prim == 0) {
-		if (line->ops.error)
-			line->ops.error(NULL, -EBADMSG);
+		if (line->ops->error)
+			line->ops->error(NULL, -EBADMSG);
 		return -EIO;
 	}
 
@@ -152,8 +152,8 @@
 		ret = e1inp_rx_ts(e1i_ts, msg, tei, sapi);
 		break;
 	default:
-		if (line->ops.error)
-			line->ops.error(NULL, -EBADMSG);
+		if (line->ops->error)
+			line->ops->error(NULL, -EBADMSG);
 		break;
 	}