fix rp2040 uart read/write non-blocking

Author: hathach

hw/bsp/rp2040/family.c

@@ -273,11 +273,13 @@ int board_uart_read(uint8_t *buf, int len) {
 
 int board_uart_write(void const *buf, int len) {
 #ifdef UART_DEV
-  char const *bufch = (char const *) buf;
-  for ( int i = 0; i < len; i++ ) {
-    uart_putc(uart_inst, bufch[i]);
+  const uint8_t *p = (const uint8_t *) buf;
+  int count = 0;
+  while (count < len && uart_is_writable(uart_inst)) {
+    uart_putc_raw(uart_inst, p[count]);
+    count++;
   }
-  return len;
+  return count;
 #else
   (void) buf; (void) len;
   return 0;