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

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 0fcb70851fbfea17 ("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.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>