net: arc: trivial: cleanup the emac driver

This patch will make the driver more readability

The emac has the error and warnings if you run
'scripts/checkpatch.pl -f --subjective xxx' to check.

Let's clean up such trivial details.

Signed-off-by: Caesar Wang <wxt@rock-chips.com>
Cc: Jiri Kosina <trivial@kernel.org>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Alexander Kochetkov <al.kochet@gmail.com>
Cc: netdev@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>