nfp: set driver VF limit

PCI subsystem has support for drivers limiting the number of VFs
available below what the IOV capability claims.  Make use of it.

While at it remove the #ifdef/#endif on CONFIG_PCI_IOV, it was
there to avoid unnecessary warnings in case device read failed
but kernel doesn't have SR-IOV support anyway.  Device reads
should not fail.

Note that we still need the driver-internal check for the case
where max VFs is 0 since PCI subsystem treats 0 as limit not set.

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