tty: fix tty_ldisc_receive_buf() documentation

The tty_ldisc_receive_buf() helper returns the number of bytes
processed so drop the bogus "not" from the kernel doc comment.

Fixes: 8d082cd3 ("tty: Unify receive_buf() code paths")
Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>