[MTD] pmc551 pci cleanup

Use pci_resource_start for getting start of regions and pci_iomap to not
doing this directly by using dev->resource... (Thanks to Rolf Eike Beer)

Signed-off-by: Jiri Slaby <jirislaby@gmail.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>