net: sh_eth: fix the rxdesc pointer when rx descriptor empty happens

When Receive Descriptor Empty happens, rxdesc pointer of the driver
and actual next descriptor of the controller may be mismatch.
This patch fixes it.

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Signed-off-by: David S. Miller <davem@davemloft.net>