net: phy: lan87xx: Decrease phy polling rate

Polling at 100Hz for 1.5s consumes quite a bit of kworker time with no
obvious benefit. Reduce that polling rate to ~6Hz.

To further save CPU and power, defer the next poll if no energy is
detected.

See: https://github.com/raspberrypi/linux/issues/4780



Signed-off-by: Phil Elwell <phil@raspberrypi.com>