Staging: otus: use ARRAY_SIZE

Change sizeof(x) / sizeof(*x) to ARRAY_SIZE(x).

Signed-off-by: Kulikov Vasiliy <segooon@gmail.com>
Acked-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>