tty: serial: document uart_get_console()

This was the only function mentioned in the text, but was neither linked
nor documented. So document and link it, so that hyperlinking works in
the text.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220728061056.20799-6-jslaby@suse.cz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>