net-sysfs: make xps_cpus_show and xps_rxqs_show consistent

Make the implementations of xps_cpus_show and xps_rxqs_show to converge,
as the two share the same logic but diverted over time. This should not
modify their behaviour but will help future changes and improve
maintenance.

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