net: dsa: mt7530: make functions mt7530_phy_write static

The function mt7530_phy_write is local to the source and does not need to
be in global scope, so make it static.

Cleans up sparse warnings:
symbol 'mt7530_phy_write' 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>