net: rtl8169: Fix return value for rtl_send_common

Return value of rtl_send_common propogates unmodified all the way
up to eth_send and further to API consumer if CONFIG_API is enabled.
Previously rtl_send_common returned number of bytes sent on success
which was erroneouly detected as error condition by API consumers
that checked for operation success by comparing return value with 0.

Switch rtl_send_common to use common convention: return 0 on success
and negative value for failure.

Cc: Stephen Warren <swarren@nvidia.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Oleksandr Tymoshenko <gonzo@bluezbox.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>