regulator: rk8xx: fix incorrect device used for _ldo_[sg]et_suspend_value

_ldo_get_suspend_value and _ldo_set_suspend_value get passed the parent
of the regulator (so the pmic) as first argument, therefore this udevice
should be used for pmic_* callbacks instead of using the parent of the
pmic.

To avoid further confusion, let's rename the argument to pmic instead of
dev, highlighting which kind of device we expect as argument.

Fixes: f047e4ab ("regulator: rk8xx: add indirection level for some ldo callbacks")
Reported-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Quentin Schulz <quentin.schulz@cherry.de>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>  # chromebook-bob