dt-bindings: serial: rs485: Fix rs485-rts-delay property

dt-bindings: serial: rs485: Fix rs485-rts-delay property

Code expects array only with 2 items which should be checked.
But also item checking is not working as it should likely because of
incorrect items description.

Fixes: d50f974c4f7f ("dt-bindings: serial: Convert rs485 bindings to json-schema")
Signed-off-by: Michal Simek <michal.simek@amd.com>
Cc: stable@vger.kernel.org
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/820c639b9e22fe037730ed44d1b044cdb6d28b75.1726480384.git.michal.simek@amd.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>