mtd: readtest: don't clobber error reports

Commit 2a6a28e7922c ("mtd: Make MTD tests cancelable") accidentally
clobbered any read failure reports.

Coverity CID #1296020

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
diff --git a/drivers/mtd/tests/readtest.c b/drivers/mtd/tests/readtest.c
index a3196b7..58df07a 100644
--- a/drivers/mtd/tests/readtest.c
+++ b/drivers/mtd/tests/readtest.c
@@ -191,9 +191,11 @@
 				err = ret;
 		}
 
-		err = mtdtest_relax();
-		if (err)
+		ret = mtdtest_relax();
+		if (ret) {
+			err = ret;
 			goto out;
+		}
 	}
 
 	if (err)