net: usb: lan78xx: move LED DT configuration to helper

Extract the LED enable logic based on the "microchip,led-modes"
property into a new helper function lan78xx_configure_leds_from_dt().

This simplifies lan78xx_phy_init() and improves modularity.
No functional changes intended.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Thangaraj Samynathan <thangaraj.s@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>