nfp: add nfp_cppcore_pcie_unit() helper

Add nfp_cppcore_pcie_unit() helper to retrieve the PCIE unit of a CPP
handle and use the new helper as appropriate.

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