mtd: spinand: toshiba: Demote non-conformant kernel-doc header

mtd: spinand: toshiba: Demote non-conformant kernel-doc header

Fixes the following W=1 kernel build warning(s):

 drivers/mtd/nand/spi/toshiba.c:36: warning: Function parameter or member 'write_cache_variants' not described in 'SPINAND_OP_VARIANTS'
 drivers/mtd/nand/spi/toshiba.c:36: warning: Function parameter or member '0' not described in 'SPINAND_OP_VARIANTS'

Cc: Miquel Raynal <miquel.raynal@bootlin.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Vignesh Raghavendra <vigneshr@ti.com>
Cc: Yoshio Furuyama <ytc-mb-yfuruyama7@kioxia.com>
Cc: Frieder Schrempf <frieder.schrempf@kontron.de>
Cc: linux-mtd@lists.infradead.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Frieder Schrempf <frieder.schrempf@kontron.de>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20201109182206.3037326-10-lee.jones@linaro.org