raid5: fix duplicate checks for rdev->saved_raid_disk

'first' will always be greater than or equal to 0, it is unnecessary to
repeat the 0 check, clean it up.

Signed-off-by: Jackie Liu <liuyun01@kylinos.cn>
Signed-off-by: Song Liu <song@kernel.org>
Signed-off-by: Jens Axboe <axboe@kernel.dk>