net: lwip: Remove error print on failed tx

net: lwip: Remove error print on failed tx

When an ethernet driver fails to send a frame we print an error in lwIP.
But depending on how often that error is it might significantly delay
transmissions.

For example downloading a big file with the rpi4 spams the console with
'send error: -101', but removing the print makes the file download with
an average speed of ~8.5MiB/s since the packets are retransmitted.

So let's move it to a 'debug' in lwIP and expect ethernet drivers to handle
the failure if it's severe.

Signed-off-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Jerome Forissier <jerome.forissier@linaro.org>