n_tty: Only process packet mode data in raw mode

Packet mode can only be set for a pty master, and a pty master is
always in raw mode since its termios cannot be changed.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Reviewed-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>