nfp: add pointer to vNIC config memory to nfp_port structure

Simplify the statistics handling code by keeping pointer to vNIC's
config memory in nfp_port.  Note that this is referring to the
representor side of vNICs, vNIC side has the pointer in nfp_net.

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