PCI: Handle case when no pci device can provide cache line size hint

Prior to this patch, if pci_read_config_byte(dev, PCI_CACHE_LINE_SIZE, ...)
returns 0 for all dev, pci_cache_line_size ends up set to zero
(instead of pci_dfl_cache_line_size).

This patch ensures the pci_cache_line_size = pci_dfl_cache_line_size
setting in the above scenario.

This happens in case of a kvm-88 guest (where, consequently, the rtl8139
NIC failed to initialize).

Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Csaba Henk <csaba@gluster.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>