PCI/IOV: Use VF0 cached config space size for other VFs

Cache the config space size from VF0 and use it for all other VFs instead
of reading it from the config space of each VF.  We assume that it will be
the same across all associated VFs.

This is an optimization when enabling SR-IOV on a device with many VFs.

Signed-off-by: KarimAllah Ahmed <karahmed@amazon.de>
[bhelgaas: use CONFIG_PCI_IOV (not CONFIG_PCI_ATS)]
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>