net: add an helper to copy xps maps to the new dev_maps

This patch adds an helper, xps_copy_dev_maps, to copy maps from dev_maps
to new_dev_maps at a given index. The logic should be the same, with an
improved code readability and maintenance.

Signed-off-by: Antoine Tenart <atenart@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>