drm/msm: Use nvmem_cell_read_variable_le_u32() to read speed bin

Let's use the newly-added nvmem_cell_read_variable_le_u32() to future
proof ourselves a little bit.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/20210521134516.v2.1.Id496c6fea0cb92ff6ea8ef1faf5d468eb09465e3@changeid


Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Rob Clark <robdclark@chromium.org>