video: fbdev: controlfb: Fix set but not used warnings

The controlfb driver has a number of dummy defines for IO operations.
They were introduced in commit a07a63b0
("video: fbdev: controlfb: add COMPILE_TEST support").

The write variants did not use their value parameter in the
dummy versions, resulting in set but not used warnings.
Fix this by adding "(void)val" to silence the compiler.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Sam Ravnborg <sam@ravnborg.org>
Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Link: https://patchwork.freedesktop.org/patch/msgid/20201206190247.1861316-13-sam@ravnborg.org