Fixes the following W=1 kernel build warning(s):

drivers/net/ethernet/dnet.c:510:6: warning:
 variable 'tx_status' set but not used [-Wunused-but-set-variable]
  u32 tx_status, irq_enable;
      ^~~~~~~~~

After commit 47964174 ("dnet: Dave DNET ethernet controller driver
(updated)"), variable 'tx_status' is never used in dnet_start_xmit(),
so removing it to avoid build warning.
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zhang Changzhong <zhangchangzhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>