drm/sun4i: dsi: Prevent underflow when computing packet sizes

In the Linux kernel, the following vulnerability has been resolved: drm/sun4i: dsi: Prevent underflow when computing packet sizes Currently, the packet overhead is subtracted using unsigned arithmetic. With a short sync pulse, this could underflow and wrap around to near the maximal u16 value. Fix this by using signed subtraction. The call to max() will correctly handle any negative numbers that are produced. Apply the same fix to the other timings, even though those subtractions are less likely to underflow.