pci: use size stored in proc_dir_entry for proc bus files

On pci_proc_attach_device(), the size of the PCI configuration space is
stored in the proc_dir_entry as the size of the file.  Thus, the procfs
interface to PCI devices should use it instead of the device directly.

Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>