video: fbdev: neofb: Fix the check of 'var->pixclock'

The previous check against 'var->pixclock' doesn't return -EINVAL when
it equals zero, but the driver uses it again, causing the divide error.

Fix this by returning when 'var->pixclock' is zero.

The following log reveals it:
 divide error: 0000 [#1] PREEMPT SMP KASAN PTI
 RIP: 0010:neofb_set_par+0x190f/0x49a0
 Call Trace:
  <TASK>
  fb_set_var+0x604/0xeb0
  do_fb_ioctl+0x234/0x670
  fb_ioctl+0xdd/0x130
  do_syscall_64+0x3b/0x90

Signed-off-by: Zheyu Ma <zheyuma97@gmail.com>
Signed-off-by: Helge Deller <deller@gmx.de>