mtd: rawnand: diskonchip: Marking unused variables as __always_unused

mtd: rawnand: diskonchip: Marking unused variables as __always_unused

'dummy' is never checked (as per the nomenclature) and the use of
'emtpymatch' is currently #if 0'ed out.  We could also #if 0 the
declaration, but #ifery is pretty ugly, so I like to keep it to a
minimum.

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

 drivers/mtd/nand/raw/diskonchip.c: In function ‘DoC_Delay’:
 drivers/mtd/nand/raw/diskonchip.c:219:16: warning: variable ‘dummy’ set but not used [-Wunused-but-set-variable]
 drivers/mtd/nand/raw/diskonchip.c: In function ‘doc200x_calculate_ecc’:
 drivers/mtd/nand/raw/diskonchip.c:706:6: warning: variable ‘emptymatch’ set but not used [-Wunused-but-set-variable]

Cc: Miquel Raynal <miquel.raynal@bootlin.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Vignesh Raghavendra <vigneshr@ti.com>
Cc: Boris Brezillon <boris.brezillon@collabora.com>
Cc: Dan Brown <dan_brown@ieee.org>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-mtd@lists.infradead.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20201109182206.3037326-16-lee.jones@linaro.org