pwm: ep93xx: Unfold legacy callbacks into ep93xx_pwm_apply()

This just puts the implementation of ep93xx_pwm_disable(),
ep93xx_pwm_enable() and ep93xx_pwm_config() into their only caller.

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