lan78xx: Don't reset the interface on open

With Alexander Graf's patch ("lan78xx: Connect phy early") applied,
the call to lan78xx_reset within lan78xx_open prevents the phy
interrupt from being generated (even though the link is up).

Avoid this issue by removing the lan78xx_reset call.

See: https://github.com/raspberrypi/linux/issues/2437
     https://github.com/raspberrypi/linux/issues/2442
     https://github.com/raspberrypi/linux/issues/2457