nfp: only try to get to PCIe ctrl memory if BARs are wide enough

For accessing PCIe ctrl memory we depend on the BAR aperture being
large enough to reach all registers.  Since the BAR aperture can
be set in the flash make sure the driver won't oops the kernel
when the PCIe configuration is unusual.

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