net: ravb: Fix R-Car RX frame size limit

The RX frame size limit should not be based on the current MTU setting.
Instead it should be based on the hardware capabilities.

While we're here, improve the description of the receive frame length
setting as suggested by Niklas.

Fixes: c156633f ("Renesas Ethernet AVB driver proper")
Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Paul Barker <paul.barker.ct@bp.renesas.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>