video: fbdev: sparc drivers: fix kernel-doc warnings for blank_mode

Fix kernel-doc warnings caused by a wrong parameter name
blank_mode => blank

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Acked-by: Thomas Zimmermann <tzimmermann@suse.de>
Cc: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20201206190247.1861316-7-sam@ravnborg.org