i3200_edac: Make a local function static

This local symbol is used only in this file.
Fix the following sparse warnings:

drivers/edac/i3200_edac.c:264:14: warning: symbol 'i3200_map_mchbar' was not declared. Should it be static?

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Borislav Petkov <bp@suse.de>