broadcom/mphalport.h: fix declaration

Author: dhalbert

ports/broadcom/mphalport.h

@@ -19,7 +19,7 @@ void mp_hal_delay_us(mp_uint_t us);
 
 void mp_hal_set_interrupt_char(int c);
 int mp_hal_stdin_rx_chr(void);
-void mp_hal_stdout_tx_strn(const char *str, size_t len);
+mp_uint_t mp_hal_stdout_tx_strn(const char *str, size_t len);
 
 #ifdef MICROPY_HW_USBHOST
 #include "usbkbd.h"