nfp: refactor NSP initialization and add error message

When acquiring NSP communication resource fails user is left with
"probe failed with error -2" PCI code message but no info on what
caused the problem.  Some development boards may not have NSP FW
in the flash image.  Help users with a more verbouse message.

While at it move the whole NSP init to a separate function to keep
.probe() callback nice and simple.

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