net: use log_err() for 'No ethernet found' message

Write the 'No ethernet found' message via the log drivers. This allows
suppressing it during output via the syslog driver.

This fixes the problem reported in:

[PATCH 0/4] log: Fix the syslog spam when running tests
https://lists.denx.de/pipermail/u-boot/2020-September/426343.html



Reported-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>