net: dsa: lan9303: make functions lan9303_mdio_phy_{read|write} static

The functions lan9303_mdio_phy_write and lan9303_mdio_phy_read are local
to the source and do not need to be in global scope, so make them static.

Cleans up sparse warnings:
symbol 'lan9303_mdio_phy_write' was not declared. Should it be static?
symbol 'lan9303_mdio_phy_read' was not declared. Should it be static?

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>