net: sh_eth: tidyup compile warning

This patch tidyup below warning

${LINUX}/drivers/net/sh_eth.c:1773: warning:
'mdp' may be used uninitialized in this function

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