ravb: Make it clear the information relates to maximum frame size

The struct member rx_max_buf_size was added before split descriptor
support was added. It is unclear if the value describes the full skb
frame buffer or the data descriptor buffer which can be combined into a
single skb.

Rename it to make it clear it referees to the maximum frame size and can
cover multiple descriptors.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Paul Barker <paul.barker.ct@bp.renesas.com>
Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>