cmd: part: Correct error handling

As 'part_get_info_by_name' now returns more status codes than just
-1 to indicate failure, we need to update the return value check.

Signed-off-by: Anders Dellien <anders.dellien@arm.com>
Reviewed-by: Sean Anderson <sean.anderson@seco.com>
diff --git a/cmd/part.c b/cmd/part.c
index 3395c17..e0463b5 100644
--- a/cmd/part.c
+++ b/cmd/part.c
@@ -140,7 +140,7 @@
 			return 1;
 	} else {
 		part = part_get_info_by_name(desc, argv[2], &info);
-		if (part == -1)
+		if (part < 0)
 			return 1;
 	}