video: fbdev: pxa168fb: make pxa168fb_init_mode() return void

No other functions use the return value of pxa168fb_init_mode() and the
return value is always 0 now. Make it return void. This fixes the
following coccicheck warning:

drivers/video/fbdev/pxa168fb.c:565:5-8: Unneeded variable: "ret". Return
"0" on line 597

Signed-off-by: Jason Yan <yanaijie@huawei.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
[fixed indent]
Link: https://patchwork.freedesktop.org/patch/msgid/20200506061745.19451-1-yanaijie@huawei.com