drm/msm/dpu: clean up dpu_kms_get_clk_rate() returns

drm/msm/dpu: clean up dpu_kms_get_clk_rate() returns

Static analysis tools complain about the -EINVAL error code being
stored in an unsigned variable.  Let's change this to match
the clk_get_rate() function which is type unsigned long and returns
zero on error.

Fixes: 25fdd5933e4c ("drm/msm: Add SDM845 DPU support")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Abhinav Kumar <quic_abhinavk@quicinc.com>
Patchwork: https://patchwork.freedesktop.org/patch/539626/
Link: https://lore.kernel.org/r/28644c5e-950e-41cd-8389-67f37b067bdc@kili.mountain
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>