FIXME: cpu/atmega_common: don't assert supported PWM mode

This makes iteration over PWM modes in the test easier.

cpu/atmega_common/periph/pwm.c

@@ -110,12 +110,15 @@ static inline void apply_config(pwm_t dev)
 
 uint32_t pwm_init(pwm_t dev, pwm_mode_t mode, uint32_t freq, uint16_t res)
 {
-    (void)mode;
     /* only left implemented, max resolution 256 */
-    assert(dev < PWM_NUMOF && mode == PWM_LEFT && res <= 256);
+    assert(dev < PWM_NUMOF && res <= 256);
     /* resolution != 256 only valid if ch0 not used */
     assert(!(res != 256 && pwm_conf[dev].pin_ch[0] != GPIO_UNDEF));
 
+    if (mode != PWM_LEFT) {
+        return 0;
+    }
+
     /* disable PWM */
 #if (defined(TCCR0A) && defined(TCCR0B)) || (defined(TCCR2A) && defined(TCCR2B))
     pwm_conf[dev].dev->CRA = 0x00;