nfp: flower: use offsets provided by pedit instead of index for ipv6

Previously when populating the set ipv6 address action, we incorrectly
made use of pedit's key index to determine which 32bit word should be
set. We now calculate which word has been selected based on the offset
provided by the pedit action.

Fixes: 354b82bb ("nfp: add set ipv6 source and destination address")
Signed-off-by: Pieter Jansen van Vuuren <pieter.jansenvanvuuren@netronome.com>
Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Signed-off-by: David S. Miller <davem@davemloft.net>