drivers: ethernet: adin2111: Add calculation of EBO in OA mode

Calculate the EBO properly, so the total frame written to TX FIFO
does not exceed maximum Ethernet frame length.
Makes the TCP stack to work properly, otherwise packets > 1472 are dopped,
as they would be padded by 64 more bytes
and hence exceed maximum Ethernet frame size.

Signed-off-by: Maciej Panek <panekmaciej@outlook.com>