net: enetc: consume the error RX buffer descriptors in a dedicated function

We can and should check the RX BD errors before starting to build the
skb. The only apparent reason why things are done in this backwards
order is to spare one call to enetc_rxbd_next.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>