amd64_edac: Mark amd64_decode_bus_error as static

This patch marks the function amd64_decode_bus_error() as static because
it is not used outside of amd64_edac.c.

It also eliminates the following warning:
drivers/edac/amd64_edac.c:2038:6: warning: no previous prototype for ‘amd64_decode_bus_error’ [-Wmissing-prototypes]

Signed-off-by: Rashika Kheria <rashika.kheria@gmail.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Link: http://lkml.kernel.org/r/7cddbd4c69ed493f183383e98853181aaf75b26b.1387029387.git.rashika.kheria@gmail.com
Signed-off-by: Borislav Petkov <bp@suse.de>