nfp: handle SR-IOV already enabled when driver is probing

We assumed that when we probe number of enabled VFs will be at 0.
This doesn't have to be the case for example if previous driver left
SR-IOV enabled due to some VFs being assigned.  Read the number of VFs
enabled.  Fail probe if it's above current FWs limit.

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>