serial: 8250: Move hp300_setup_serial_console() to <linux/serial_8250.h>

If CONFIG_SERIAL_8250_HP300=y and CONFIG_SERIAL_8250_CONSOLE=y (e.g.
m68k/allyesconfig):

    drivers/tty/serial/8250/8250_hp300.c:91:12: error: no previous prototype for ‘hp300_setup_serial_console’ [-Werror=missing-prototypes]
       91 | int __init hp300_setup_serial_console(void)
	  |            ^~~~~~~~~~~~~~~~~~~~~~~~~~

Fix this by moving the existing prototype in arch/m68k/hp300/config.c to
<linux/serial_8250.h>, so it is visible to both caller and implementor.
While at it, provide a dummy in case CONFIG_SERIAL_8250_CONSOLE is not
enabled, to reduce #ifdef clutter in the caller.

Exposed by commit 0fcb7085 ("Makefile.extrawarn: turn on
missing-prototypes globally").
Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/c17469f8e47b2ef49234a85a7a14882ddf374e41.1704892597.git.geert@linux-m68k.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/m68k/hp300/config.c

@@ -10,6 +10,7 @@
 
 #include <linux/module.h>
 #include <linux/init.h>
+#include <linux/serial_8250.h>
 #include <linux/string.h>
 #include <linux/kernel.h>
 #include <linux/console.h>
@@ -67,9 +68,6 @@ static char *hp300_models[] __initdata = {
 static char hp300_model_name[13] = "HP9000/";
 
 extern void hp300_reset(void);
-#ifdef CONFIG_SERIAL_8250_CONSOLE
-extern int hp300_setup_serial_console(void) __init;
-#endif
 
 int __init hp300_parse_bootinfo(const struct bi_record *record)
 {
@@ -263,7 +261,5 @@ void __init config_hp300(void)
 	} else {
 		panic("Unknown HP9000 Model");
 	}
-#ifdef CONFIG_SERIAL_8250_CONSOLE
 	hp300_setup_serial_console();
-#endif
 }

include/linux/serial_8250.h

@@ -210,6 +210,12 @@ int serial8250_console_exit(struct uart_port *port);
 void serial8250_set_isa_configurator(void (*v)(int port, struct uart_port *up,
 					       u32 *capabilities));
 
+#ifdef CONFIG_SERIAL_8250_CONSOLE
+extern int hp300_setup_serial_console(void) __init;
+#else
+static inline int hp300_setup_serial_console(void) { return 0; }
+#endif
+
 #ifdef CONFIG_SERIAL_8250_RT288X
 int rt288x_setup(struct uart_port *p);
 int au_platform_setup(struct plat_serial8250_port *p);