drivers: eth: native_posix: Don't sleep when there is data to read

Don't sleep 50ms after each received packet, sleep only when there
wasn't anything to receive. Otherwise data could get stuck for a long
time if there was more than 20 packets coming in per second. The
read() call on a TUN/TAP device returns only a single packet per call.

Also remove the call to eth_stats_update_errors_rx() because this else
clause isn't actually a receive error, we're just waiting for more
packets.

Signed-off-by: Jonas Norling <jonas.norling@greeneggs.se>