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.

Fixes: 81dee67 ("staging: sm750fb: add sm750 to staging")
Reported-by: Yuhao Jiang <danisjiang@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Junrui Luo <moonafterrain@outlook.com>
Link: https://patch.msgid.link/SYBPR01MB7881AFBFCE28CCF528B35D0CAF4BA@SYBPR01MB7881.ausprd01.prod.outlook.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: avasummer

drivers/staging/sm750fb/sm750.c

@@ -481,6 +481,9 @@ static int lynxfb_ops_check_var(struct fb_var_screeninfo *var,
 	struct lynxfb_crtc *crtc;
 	resource_size_t request;
 
+	if (!var->pixclock)
+		return -EINVAL;
+
 	ret = 0;
 	par = info->par;
 	crtc = &par->crtc;