net: ibm: remove set but not used variables 'err'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/net/ethernet/ibm/emac/core.c: In function __emac_mdio_write:
drivers/net/ethernet/ibm/emac/core.c:875:9: warning:
	variable err set but not used [-Wunused-but-set-variable]

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Chen Zhou <chenzhou10@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>