pwm: stmpe: Handle errors when disabling the signal

Before the pwm framework implementedatomic updates (with the .apply()
callback) the .disable() callback returned void. This is still visible
in the stmpe driver which drops errors in the disable path.

Improve the driver to forward failures in stmpe_24xx_pwm_disable() to
the caller of pwm_apply_state().

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>