regulator: max77686: Use already set opmode while re-enabling regulators.

This patch adds basic support to take care of opmode(if any) which can be set by
set_suspend_[disable/mode] callbacks.

Signed-off-by: Yadwinder Singh Brar <yadi.brar@samsung.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>