net: phy: smsc: skip ENERGYON interrupt if disabled

Don't enable the interrupt if the platform disable the energy detection
by "smsc,disable-energy-detect".

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>