net: sun8i_emac: Simplify and fix error handling for RX

The error handling in recv() is somewhat broken, for instance
good_packet isn't really used, and it's hardly readable. Also we try
to check for short or too big packets, but those are actually filtered
out by the hardware.

Simplify the whole routine and improve the error handling:
- Bail out early if the current RX descriptor is not ready.
- Enable propagation of runt, huge and broken packets.
- Check for runt and huge packets, and return 0 to indicate this.
  This will force the framework to call free_pkt for cleanup.
- Avoid aligning the packet buffer for invalidation again.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Tested-by: Amit Singh Tomar <amittomer25@gmail.com> # Pine64+
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>