regulator: tps6586x: Add missing bit mask generation

Change-Id: I76eaceb31b56264f6978af15db1e6fc7e2e01b5a
Signed-off-by: Danny Huang <dahuang@nvidia.com>
Signed-off-by: Olof Johansson <olofj@chromium.org>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
  (Split into separate patches)
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>