net: ravb: Refactor GbEth RX code path

We can reduce code duplication in ravb_rx_gbeth().

Signed-off-by: Paul Barker <paul.barker.ct@bp.renesas.com>
Reviewed-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>