sh_eth: fix unused variable warning

Commit d5e07e69 (sh_eth: use managed device API)
has caused this warning (due to my overlook):

drivers/net/ethernet/renesas/sh_eth.c: In function `sh_eth_drv_remove':
drivers/net/ethernet/renesas/sh_eth.c:2482:25: warning: unused variable `mdp'
[-Wunused-variable]

Kill the darn variable now...

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>