kselftest/arm64: Supress warning and improve readability

The comment was correct, but `checkpatch` script flagged it with a warning
as shown in the output section. The comment is slightly modified
to improve readability, which also suppresses the warning.

```
[command]
./script/checkpatch.pl --strict -f tools/testing/selftests/arm64/fp/fp-stress.c

[output]
WARNING: Possible repeated word: 'on'
```

Signed-off-by: Vivek Yadav <[e-mail]>
Signed-off-by: Will Deacon <[e-mail]>

tools/testing/selftests/arm64/fp/fp-stress.c

···
 
 		/*
 		 * Read from the startup pipe, there should be no data
-		 * and we should block until it is closed.  We just
-		 * carry on on error since this isn't super critical.
+		 * and we should block until it is closed. We just
+		 * carry-on on error since this isn't super critical.
 		 */
 		ret = read(3, &i, sizeof(i));
 		if (ret < 0)