MAINTAINERS: update email for Dan Williams

Returned to intel.com

Cc: Vinod Koul <vinod.koul@intel.com>
Cc: Linus Walleij <linus.walleij@linaro.org>
Cc: Jon Mason <jon.mason@intel.com>
Cc: Dave Jiang <dave.jiang@intel.com>
Cc: Neil Brown <neilb@suse.de>
Cc: Shaohua Li <shli@kernel.org>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>