NRF52480: fix Serial TX/RX pins

This commit should be later reverted, and master version kept.

Author: 0xc0170

targets/TARGET_NORDIC/TARGET_NRF5/TARGET_MCU_NRF52840/PeripheralNames.h

@@ -46,11 +46,11 @@ extern "C" {
 #endif
 
 #ifndef STDIO_UART_TX
-#define STDIO_UART_TX     TX_PIN_NUMBER
+#define STDIO_UART_TX     USBTX
 #endif
 
 #ifndef STDIO_UART_RX
-#define STDIO_UART_RX     RX_PIN_NUMBER
+#define STDIO_UART_RX     USBRX
 #endif
 
 #define STDIO_UART        UART_0