net: phy: export phy_start_machine() for phylink

phylink will need phy_start_machine exported, so lets export it as a
GPL symbol.  Documentation/networking/phy.txt indicates that this
should be a PHY API function.

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>