nrf52820_microbit: Reduce CDC buffer size to 64B to reduce WebUSB lost chars.
Issue discussion:
#define USBD_CDC_ACM_EP_BULKIN_STACK 0
#define USBD_CDC_ACM_WMAXPACKETSIZE1 64
#define USBD_CDC_ACM_HS_ENABLE1 1
-#define USBD_CDC_ACM_HS_WMAXPACKETSIZE1 512
+#define USBD_CDC_ACM_HS_WMAXPACKETSIZE1 64
#define USBD_CDC_ACM_HS_BINTERVAL1 1
#define USBD_CDC_ACM_CIF_STRDESC L"mbed Serial Port"
#define USBD_CDC_ACM_DIF_STRDESC L"mbed Serial Port"
