native/platform: fix DFU detach

DFU detach wasn't lowering the USB pullup. This meant that BMP was
rebooting into DFU bootloader mode, but the host wasn't re-enumerating
it, because it hadn't detected a detach.

In `platform_request_boot`, The `gpio_set_mode` config was for analog
instead of floating input. PA8 isn't defined as an analog pin on this
chip, so it's arguably undefined behavior.

Also confirmed that using `scb_reset_system` instead of `scb_reset_core`
to do the DFU reboot would correctly detach and enter the bootloader, if
the boot button was held down.

src/platforms/native/platform.c

@@ -414,7 +414,7 @@ const char *platform_target_voltage(void)
 void platform_request_boot(void)
 {
 	/* Disconnect USB cable */
-	gpio_set_mode(USB_PU_PORT, GPIO_MODE_INPUT, 0, USB_PU_PIN);
+	gpio_set_mode(USB_PU_PORT, GPIO_MODE_INPUT, GPIO_CNF_INPUT_FLOAT, USB_PU_PIN);
 
 	/* Drive boot request pin */
 	gpio_set_mode(GPIOB, GPIO_MODE_OUTPUT_2_MHZ, GPIO_CNF_OUTPUT_PUSHPULL, GPIO12);