sisfb: delete fallback code for pci_map_rom()

If pci_map_rom() fails, there is some fallback code that basically
duplicates pci_map_rom() on non-x86 platforms. No point in that.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Cc: Thomas Winischhofer <thomas@winischhofer.net>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>