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 <vivekyadav1207731111@gmail.com>
Signed-off-by: Will Deacon <will@kernel.org>