mtd: core: Constify buf in mtd_write_user_prot_reg()

The write buffer comes from user and should be const.
Constify write buffer in mtd core and across all _write_user_prot_reg()
users. cfi_cmdset_{0001, 0002} and onenand_base will pay the cost of an
explicit cast to discard the const qualifier since the beginning, since
they are using an otp_op_t function prototype that is used for both reads
and writes. mtd_dataflash and SPI NOR will benefit of the const buffer
because they are using different paths for writes and reads.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210403060931.7119-1-tudor.ambarus@microchip.com