mfd: ab8500-core: 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>
Signed-off-by: Lee Jones <lee.jones@linaro.org>