do_ping() expects ping_loop() to return a negative value on error, so
that it can propagate it to the caller as CMD_RET_FAILURE. This is not
the case when no ethernet device is found, so fix that.
Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
(cherry picked from commit 42df30a210)