serial: serial_irq() and descendants can be __init

... as being solely called from smp_intr_init(), which itself is
marked such.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/drivers/char/ns16550.c
+++ b/xen/drivers/char/ns16550.c
@@ -361,7 +361,7 @@ static void __init ns16550_endboot(struc
 #define ns16550_endboot NULL
 #endif
 
-static int ns16550_irq(struct serial_port *port)
+static int __init ns16550_irq(struct serial_port *port)
 {
     struct ns16550 *uart = port->uart;
     return ((uart->irq > 0) ? uart->irq : -1);
--- a/xen/drivers/char/pl011.c
+++ b/xen/drivers/char/pl011.c
@@ -215,7 +215,7 @@ static int pl011_getc(struct serial_port
     return 1;
 }
 
-static int pl011_irq(struct serial_port *port)
+static int __init pl011_irq(struct serial_port *port)
 {
     struct pl011 *uart = port->uart;
     return ((uart->irq > 0) ? uart->irq : -1);
--- a/xen/drivers/char/serial.c
+++ b/xen/drivers/char/serial.c
@@ -475,7 +475,7 @@ void __init serial_endboot(void)
             com[i].driver->endboot(&com[i]);
 }
 
-int serial_irq(int idx)
+int __init serial_irq(int idx)
 {
     if ( (idx >= 0) && (idx < ARRAY_SIZE(com)) &&
          com[idx].driver && com[idx].driver->irq )