core: serial: fix availableForWrite()

availableForWrite() was taking the free space of the Rx buffer
instead of the Tx buffer.

Author: silabs-bozont

cores/arduino/zephyrSerial.cpp

@@ -116,6 +116,16 @@ int arduino::ZephyrSerial::available() {
 	return ret;
 }
 
+int arduino::ZephyrSerial::availableForWrite() {
+	int ret;
+
+	k_sem_take(&tx.sem, K_FOREVER);
+	ret = ring_buf_space_get(&tx.ringbuf);
+	k_sem_give(&tx.sem);
+
+	return ret;
+}
+
 int arduino::ZephyrSerial::peek() {
 	uint8_t data;