dpaa2-switch: reorganize the [pre]changeupper events

Create separate functions, dpaa2_switch_port_prechangeupper and
dpaa2_switch_port_changeupper, to be called directly when a DPSW port
changes its upper device.

This way we are not open-coding everything in the main event callback
and we can easily extent, for example, with bond offload.

Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>