nfp: add nfp_net_pf_free_vnic() function

Soon a third place will need to free a struct nfp_net.  Add a free
counterpart to nfp_net_pf_alloc_vnic().

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>