MIPS: PCI: Prevent hang on XLP reg read

Reading PCI extended register at 0x255 on a bridge will hang if there
is no device connected on the link. Make PCI read routine skip this
register.

Signed-off-by: Jayachandran C <jchandra@broadcom.com>
Patchwork: http://patchwork.linux-mips.org/patch/4789/
Signed-off-by: John Crispin <blogic@openwrt.org>