Below cases are supposed to be valid:

min_uV == max_uV == info->max_uV
min_uV == max_uV == rdev->desc->min_uV

Don't return -EINVAL for above cases.

This patch also includes below cleanups:
- Use rdev_get_drvdata(rdev) instead of rdev->reg_data.
- Remove unnecessary WARN_ON, it looks pointless.
Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>