tty/serial: mark __init early_smh_setup() static

We get 1 warning when building kernel with W=1:
drivers/tty/serial/earlycon-arm-semihost.c:56:12: warning: no previous prototype for 'early_smh_setup' [-Wmissing-prototypes]

In fact, this function is only used in the file in which it is
declared and don't need a declaration, but can be made static.
so this patch marks this function with 'static'.
Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/earlycon-arm-semihost.c

@@ -53,7 +53,8 @@ static void smh_write(struct console *con, const char *s, unsigned n)
 	uart_console_write(&dev->port, s, n, smh_putc);
 }
 
-int __init early_smh_setup(struct earlycon_device *device, const char *opt)
+static int
+__init early_smh_setup(struct earlycon_device *device, const char *opt)
 {
 	device->con->write = smh_write;
 	return 0;