ARM: rmobile: Clean up rmobile_cpuinfo_idx()

Clean the function up a bit further. Return immediatelly on match
and return ARRAY_SIZE() - 1 on failure. Add proper comment in that
case.

Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Paul Barker <paul.barker.ct@bp.renesas.com>