staging: sm750fb: fix division by zero in ps_to_hz()

In the Linux kernel, the following vulnerability has been resolved:

staging: sm750fb: fix division by zero in ps_to_hz()

ps_to_hz() is called from hw_sm750_crtc_set_mode() without validating

that pixclock is non-zero. A zero pixclock passed via FBIOPUT_VSCREENINFO

causes a division by zero.

Fix by rejecting zero pixclock in lynxfb_ops_check_var(), consistent

with other framebuffer drivers.