sunhme: fix return values from sunhme_receive() during receive packet processing

sunhme: fix return values from sunhme_receive() during receive packet processing

The current return values in sunhme_receive() when processing incoming packets
are inverted from what they should be. Make sure that we return 0 to indicate
the packet was discarded (and polling is to be disabled) and -1 to indicate
that the packet was discarded but polling for incoming data is to be continued.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>