xen: update tx_ready callback for ARM serial drivers

Type of tx_ready callback got changed to int to facilitate error condition,
but the ARM serial drivers were not modified thus breaking the compilation.

Reported-by: Julien Grall <julien.grall@linaro.org>
Signed-off-by: Tomasz Wroblewski <tomasz.wroblewski@citrix.com>

xen/drivers/char/exynos4210-uart.c

@@ -221,7 +221,7 @@ static void exynos4210_uart_resume(struct serial_port *port)
     BUG(); // XXX
 }
 
-static unsigned int exynos4210_uart_tx_ready(struct serial_port *port)
+static int exynos4210_uart_tx_ready(struct serial_port *port)
 {
     struct exynos4210_uart *uart = port->uart;
 

xen/drivers/char/omap-uart.c

@@ -226,7 +226,7 @@ static void omap_uart_resume(struct serial_port *port)
     BUG();
 }
 
-static unsigned int omap_uart_tx_ready(struct serial_port *port)
+static int omap_uart_tx_ready(struct serial_port *port)
 {
     struct omap_uart *uart = port->uart;
     uint32_t reg;

xen/drivers/char/pl011.c

@@ -155,7 +155,7 @@ static void pl011_resume(struct serial_port *port)
     BUG(); // XXX
 }
 
-static unsigned int pl011_tx_ready(struct serial_port *port)
+static int pl011_tx_ready(struct serial_port *port)
 {
     struct pl011 *uart = port->uart;