mtd: core: use MTD_DEVICE_ATTR_RO/RW() helper macros

Use MTD_DEVICE_ATTR_RO/RW() helper macros 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: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210603125323.12142-3-thunder.leizhen@huawei.com