nfp: turn reading PCIe RTsym parameters into a helper

Turn the function to read number of ports into a generic helper.
While at it make sure we propagate all errors other than -ENOENT.

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