usbnet: rx_submit() should return an error code.

This patch makes rx_submit() return an error code, and makes some call sites
that care check the return value. This is important because it lets us properly
handle cases where the device isn't ready to handle URB submissions (e.g., when
it is autosuspended under some drivers); previously, we would attempt and fail
to submit URBs and reschedule ourselves to try and fail again. This patch is
against Linus's 2.6 repo commit 45d7f32c.

Signed-Off-By: Elizabeth Jones <ellyjones@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>