net: sun4i-emac: Staticize local symbols

Some symbols referenced only in this file are made static.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Cc: Stefan Roese <sr@denx.de>
Cc: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>