regulator: userspace-consumer: use DEVICE_ATTR_RO/RW macro

Use DEVICE_ATTR_RO/RW macro helper instead of plain DEVICE_ATTR, which
makes the code a bit shorter and easier to read.

Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Link: https://lore.kernel.org/r/20210602080526.11117-1-thunder.leizhen@huawei.com


Signed-off-by: Mark Brown <broonie@kernel.org>