net: lan78xx: Allow for VLAN headers in timeout.

The frame abort timeout being set by lan78xx_set_rx_max_frame_length
didn't account for any VLAN headers, resulting in very low
throughput if used with tagged VLANs.
Use VLAN_ETH_HLEN instead of ETH_HLEN to correct for this.

See https://github.com/raspberrypi/linux/issues/2458



Signed-off-by: Dave Stevenson <dave.stevenson@raspberrypi.org>