nrf52820_microbit: Reduce CDC buffer size to 64B to reduce WebUSB lost chars.

Issue discussion: ARMmbed#903

Author: jaustin

source/hic_hal/nordic/nrf52820/usb_config.c

@@ -340,7 +340,7 @@
 #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"