tty: cleanup tty_chars_in_buffer

There is no need for 'else' when the 'if' part already returned. This
makes tty_chars_in_buffer similar to tty_write_room too.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20210302062214.29627-42-jslaby@suse.cz


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