pwm: stmpe: Rename variable pointing to driver private data

In all code locations but the probe function variables of type struct
stmpe_pwm * are called "stmpe_pwm". Align the name used in
stmpe_pwm_probe() accordingly. Still more as the current name "pwm" is
usually reserved for variables of type struct pwm_device *.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>