raspberrypi: PDMIn: Fix sample rate check.

The "actual frequency" is the PIO frequency, which is twice the audio
clock.

Author: jepler

ports/raspberrypi/common-hal/audiobusio/PDMIn.c

@@ -64,7 +64,7 @@ void common_hal_audiobusio_pdmin_construct(audiobusio_pdmin_obj_t *self,
         PIO_FIFO_TYPE_DEFAULT,
         PIO_MOV_STATUS_DEFAULT, PIO_MOV_N_DEFAULT);
     uint32_t actual_frequency = common_hal_rp2pio_statemachine_get_frequency(&self->state_machine);
-    if (actual_frequency < MIN_MIC_CLOCK) {
+    if (actual_frequency < 2 * MIN_MIC_CLOCK) { // 2 PIO samples per audio clock
         mp_raise_ValueError(MP_ERROR_TEXT("sampling rate out of range"));
     }