x38_edac: Make a local function static

Make a local function static in order to fix the following sparse
warning:

drivers/edac/x38_edac.c:252:14: warning: symbol 'x38_map_mchbar' was not declared. Should it be static?

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
[ Boris: Correct commit message ]
Signed-off-by: Borislav Petkov <bp@suse.de>