drm/msm/dpu: tidy up some error checking

The "vsync_hz" variable is unsigned int so it can't be less
than zero.  The dpu_kms_get_clk_rate() function used to return a u64
but I previously changed it to return an unsigned long and zero on
error so it matches clk_get_rate().

Change the "vsync_hz" type to unsigned long as well and change the
error checking to check for zero instead of negatives.  This change
does not affect runtime at all.  It's just a clean up.

Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Reviewed-by: Marijn Suijten <marijn.suijten@somainline.org>
Patchwork: https://patchwork.freedesktop.org/patch/541225/
Link: https://lore.kernel.org/r/ZH7vP2Swu8CYpgUL@moroto


[DB: fixed debug message]
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>