PCI: Print more info in sriov_enable() error message

If we don't have space for all the bus numbers required to enable VFs,
print the largest bus number required and the range available.

No functional change; improved error message only.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Wei Yang <weiyang@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>