net: dsa: allow rcv() and xmit() to be optional

net: dsa: allow rcv() and xmit() to be optional

Allow rcv() and xmit() dsa driver ops to be optional in case a driver
does not care to mangle a packet as in U-Boot only one network port is
enabled at a time and thus no packet mangling is necessary.

Suggested-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Tim Harvey <tharvey@gateworks.com>