net: phy: export phy_check_valid() function

This function will be needed for genphy_c45_ethtool_get_eee() provided
by next patch.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
Signed-off-by: David S. Miller <davem@davemloft.net>