nfp: fail graciously when someone tries to grab global lock

The global device lock is acquired to search the resource table.
The lock is actually itself part of the table (entry 0).
Therefore if someone asks for resource 0 we would deadlock since
double locking is no longer allowed.

Currently the driver doesn't try to lock that resource so let's
simply make sure we fail graciously and not add special handling
of this case until really need.  Hide the relevant defines in
the source file.

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