ravb: Add rx_max_buf_size to struct ravb_hw_info

R-Car AVB-DMAC has maximum 2K size on RX buffer, whereas on RZ/G2L
it is 8K. We need to allow for changing the MTU within the limit
of the maximum size of a descriptor.

Add a rx_max_buf_size variable to struct ravb_hw_info to handle
this difference.

Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>