Fix F303 audio output on A4 with the dac_basic driver (qmk#12480)

The dac_basic driver did not work properly with `#define AUDIO_PIN A4`
(instead of configuring the A4 pin, the driver actually was switching
the A5 pin to analog mode, breaking any other usage of that pin in
addition to emitting a distorted signal on the improperly configured
A4 pin).  Fix the code to configure the A4 pin as intended.

quantum/audio/driver_chibios_dac_basic.c

@@ -101,7 +101,7 @@ static const DACConversionGroup dac_conv_grp_ch2 = {.num_channels = 1U, .trigger
 void channel_1_start(void) {
  gptStart(&GPTD6, &gpt6cfg1);
  gptStartContinuous(&GPTD6, 2U);
- palSetPadMode(GPIOA, 5, PAL_MODE_INPUT_ANALOG);
+ palSetPadMode(GPIOA, 4, PAL_MODE_INPUT_ANALOG);
 }
 
 void channel_1_stop(void) {