pwm: atmel-tcb: Unroll atmel_tcb_pwm_set_polarity() into only caller

atmel_tcb_pwm_set_polarity() is only called once and effectively wraps
an assignment only. Replace the function call by the respective
assignment.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Claudiu Beznea <claudiu.beznea@tuxon.dev>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>