video: fbdev: arkfb: Fix a divide-by-zero bug in ark_set_pixclock()

Since the user can control the arguments of the ioctl() from the user
space, under special arguments that may result in a divide-by-zero bug
in:
  drivers/video/fbdev/arkfb.c:784: ark_set_pixclock(info, (hdiv * info->var.pixclock) / hmul);
with hdiv=1, pixclock=1 and hmul=2 you end up with (1*1)/2 = (int) 0.
and then in:
  drivers/video/fbdev/arkfb.c:504: rv = dac_set_freq(par->dac, 0, 1000000000 / pixclock);
we'll get a division-by-zero.

The following log can reveal it:

divide error: 0000 [#1] PREEMPT SMP KASAN PTI
RIP: 0010:ark_set_pixclock drivers/video/fbdev/arkfb.c:504 [inline]
RIP: 0010:arkfb_set_par+0x10fc/0x24c0 drivers/video/fbdev/arkfb.c:784
Call Trace:
 fb_set_var+0x604/0xeb0 drivers/video/fbdev/core/fbmem.c:1034
 do_fb_ioctl+0x234/0x670 drivers/video/fbdev/core/fbmem.c:1110
 fb_ioctl+0xdd/0x130 drivers/video/fbdev/core/fbmem.c:1189

Fix this by checking the argument of ark_set_pixclock() first.

Fixes: 681e1473

 ("arkfb: new framebuffer driver for ARK Logic cards")
Signed-off-by: Zheyu Ma <zheyuma97@gmail.com>
Signed-off-by: Helge Deller <deller@gmx.de>