mtd: spinand: micron: Use more specific names

Rename the read/write/update of SPINAND_OP_VARIANTS() to more
specialized names.

Signed-off-by: Thirumalesha Narasimhappa <nthirumalesha7@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20201108113735.2533-2-nthirumalesha7@gmail.com