net: dsa: microchip: enable phy interrupts only if interrupt enabled in dts

In the lan937x_mdio_register function, phy interrupts are enabled
irrespective of irq is enabled in the switch. Now, the check is added to
enable the phy interrupt only if the irq is enabled in the switch.

Signed-off-by: Arun Ramadoss <arun.ramadoss@microchip.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>