EDAC, ghes: Use BIT() macro

... for improved readability. Also, add a local mask variable for the
same reason.

No functional changes.

Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Toshi Kani <toshi.kani@hpe.com>
Cc: Mauro Carvalho Chehab <mchehab@kernel.org>