net: sh_eth: Fix RX error handling

In case RX error occurs, and the RD_RFE bit is set, the descriptor
is never returned back to the queue. Make sh_eth_recv_start return
zero length in this case so that the descriptor can be released
and pushed back to the list. Also return the more appropriate
-EAGAIN instead of -EINVAL if the descriptor is not ready yet.

Signed-off-by: Valentine Barshak <valentine.barshak@cogentembedded.com>
Reviewed-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>