net: dpaa2-switch: refactor prechangeupper sanity checks

Make more room for some extra code in the NETDEV_PRECHANGEUPPER handler
by moving what already exists into a dedicated function.

Cc: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>