serial: amba-pl011: Use __releases/__acquires annotations

Fix the following sparse warnings:

drivers/tty/serial/amba-pl011.c:687:20: warning: context imbalance in 'pl011_dma_flush_buffer' - unexpected unlock
drivers/tty/serial/amba-pl011.c:1200:13: warning: context imbalance in 'pl011_rx_chars' - unexpected unlock
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/amba-pl011.c

@@ -677,6 +677,8 @@ static inline bool pl011_dma_tx_start(struct uart_amba_port *uap)
  * Locking: called with port lock held and IRQs disabled.
  */
 static void pl011_dma_flush_buffer(struct uart_port *port)
+__releases(&uap->port.lock)
+__acquires(&uap->port.lock)
 {
 	struct uart_amba_port *uap = (struct uart_amba_port *)port;
 
@@ -1198,6 +1200,8 @@ static void pl011_enable_ms(struct uart_port *port)
 }
 
 static void pl011_rx_chars(struct uart_amba_port *uap)
+__releases(&uap->port.lock)
+__acquires(&uap->port.lock)
 {
 	pl011_fifo_to_tty(uap);