pwm: atmel-tcb: Drop duplicated tracking of per-channel data

Per-channel data is tracked using struct pwm_device::chip_data and
struct atmel_tcb_pwm_chip::pwms[]. Simplify by using the latter
consistently.

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