thermal: rcar_thermal: Use usleep_range() instead of udelay()

rcar_thermal_update_temp() takes a mutex, so it is always called in a
context that can sleep.  Hence replace the 300 µs busy loop by a call to
usleep_range(), to allow other threads to run.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200115125417.5263-1-geert+renesas@glider.be