lib/utils/stdout_helpers: Fix function signature to match py/mphal.h.

lib/utils/stdout_helpers.c

@@ -11,7 +11,7 @@
 
 // Send "cooked" string of given length, where every occurance of
 // LF character is replaced with CR LF.
-void mp_hal_stdout_tx_strn_cooked(const char *str, mp_uint_t len) {
+void mp_hal_stdout_tx_strn_cooked(const char *str, size_t len) {
     while (len--) {
         if (*str == '\n') {
             mp_hal_stdout_tx_strn("\r", 1);