When using the serial console over USB to download files, and
when the USB bandwidth is pushed to its limits, the barebox UDC
device won't be able to absorb all the traffic.

Modify the u_serial gadget so that if there is not enough room
in buffers, don't push USB requests down the gadget driver, so
that it is saturated, and give it a chance to NAK requests.

The previous behaviour was loosing bytes (as kfifo_put is lossy).
The fixed behavious is lossless (based on USB NAK protocol).

While at it, increase a bit buffer sizes to 8kB to absorb heavy
transfers such as a linux kernel image.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>