nfp: explicitly check if application FW is loaded

We support application FW being either loaded automatically at
boot from flash or (more commonly) by the driver from disk.
If FW is not found on disk and nothing is preloaded users are
faced with this unintuitive error:

nfp 0000:04:00.0: nfp: Failed to find PF symbol _pf0_net_bar0

We can do better.  Since we rely on symbol table being present -
check early if it could be correctly read out of from the device
and if not print a more informative message.

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>