pci: clean up of pci_default_read_config.

This patch cleans up pci_default_read_config() removing
ugly length and range check.

Suggested by "Michael S. Tsirkin" <mst@redhat.com>
Signed-off-by: Isaku Yamahata <yamahata@valinux.co.jp>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
