pwm: vt8500: Rename pwm_busy_wait() to make it obviously driver-specific

The pwm_ prefix suggests that pwm_busy_wait() is a function provided by
the pwm core. Use the otherwise consistently used driver prefix for this
function, too.

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