pwm: renesas-tpu: Rename variables to match the usual naming

The driver used "pwm" for struct tpu_pwm_device pointers. This name is
usually only used for struct pwm_device pointers which this driver calls
"_pwm". So rename to the driver data pointers to "tpd" which then allows
to drop the underscore from "_pwm".

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>