regulator: core: Use DEVICE_ATTR_RO macro

Use DEVICE_ATTR_RO() helper instead of plain DEVICE_ATTR(),
which makes the code a bit shorter and easier to read.
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Link: https://lore.kernel.org/r/20210529115226.25376-1-yuehaibing@huawei.comSigned-off-by: Mark Brown <broonie@kernel.org>