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

@@ -126,9 +126,9 @@ int arduino::ZephyrSerial::available() {
 int arduino::ZephyrSerial::availableForWrite() {
 	int ret;
 
-	k_sem_take(&rx.sem, K_FOREVER);
-	ret = ring_buf_space_get(&rx.ringbuf);
-	k_sem_give(&rx.sem);
+	k_sem_take(&tx.sem, K_FOREVER);
+	ret = ring_buf_space_get(&tx.ringbuf);
+	k_sem_give(&tx.sem);
 
 	return ret;
 }