ravb: Move maximum Rx descriptor data usage to info struct

To make it possible to merge the R-Car and RZ/G2L code paths move the
maximum usable size of a single Rx descriptor data slice into the
hardware information instead of using two different defines in the two
different code paths.

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>