mtd: spinand: winbond: Convert W25N specific operation to SPINAND_OP()

mtd: spinand: winbond: Convert W25N specific operation to SPINAND_OP()

Winbond W25N* chips require a vendor specific operation to select the
target. Instead of defining this op only in the function that
needs it, hiding it from the core, make it a proper define like all
other spi-mem operations, and implement the necessary
spinand_fill_*_op() helper to make the SPINAND_OP() macro work. This way
we can use it from any function without any extra handling outside of
this helper when we will convert the core to support octal DDR busses.

Reviewed-by: Tudor Ambarus <tudor.ambarus@linaro.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>