mtd: readtest: don't clobber error reports

Commit 2a6a28e7 ("mtd: Make MTD tests cancelable") accidentally
clobbered any read failure reports.

Coverity CID #1296020
Signed-off-by: NBrian Norris <computersforpeace@gmail.com>

drivers/mtd/tests/readtest.c

@@ -191,9 +191,11 @@ static int __init mtd_readtest_init(void)
 				err = ret;
 		}
 
-		err = mtdtest_relax();
-		if (err)
+		ret = mtdtest_relax();
+		if (ret) {
+			err = ret;
 			goto out;
+		}
 	}
 
 	if (err)