n_tty: remove superfluous return from n_tty_receive_signal_char

A return at the end of a void-returning function is superfluous. Get rid
of it.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210505091928.22010-11-jslaby@suse.cz


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