drivers: ethernet: adin2111: move OA buffers out from device data

Move OA-related TX and RX buffers out from the device data.
Don't define the buffers without the `spi-oa` property. That
saves ~32KB when using generic spi protocol.

Rearrange device data fields to reduce byte holes found with
Pahole.

Signed-off-by: Georgij Cernysiov <geo.cgv@gmail.com>