nfp: move RSS indirection table init into a separate function

We will need to rerun the initialization of the RSS indirection table
after the number of rings is changed.  Move the code to a separate
function.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>