net: arc_emac: mark arc_mdio_reset() static

We get 1 warning when building kernel with W=1:
drivers/net/ethernet/arc/emac_mdio.c:107:5: warning: no previous prototype for 'arc_mdio_reset' [-Wmissing-prototypes]

In fact, this function is only used in the file in which it is
declared and don't need a declaration, but can be made static.
so this patch marks this function with 'static'.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>