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

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CVE-2026-31603

CVE-2026-31603 affects the Linux kernel staging: sm750fb driver. The issue occurs when a zero pixclock is passed via FBIOPUT_VSCREENINFO, causing ps_to_hz() to divide by zero in hw_sm750_crtc_set_mode(). The vulnerability is resolved by rejecting zero pixclock in lynxfb_ops_check_var(), aligning ...