staging: sm750fb: add missing case while setting FB_VISUAL

Switch statement does not contain all cases: 8, 16, 24, 32.
This patch will add missing one (24)

Fixes: 81dee67e ("staging: sm750fb: add sm750 to staging")
Signed-off-by: Matej Dujava <mdujava@kocurkovo.cz>
Link: https://lore.kernel.org/r/1588277366-19354-2-git-send-email-mdujava@kocurkovo.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>