sh_eth: Enable Rx descriptor word 0 shift for r8a7790

This corrects an oversight when r8a7790 support was added to sh_eth.

Signed-off-by: Kouei Abe <kouei.abe.cp@renesas.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: David S. Miller <davem@davemloft.net>