n_tty: Special case EXTPROC receive_buf() as raw mode

When EXTPROC is set without ISTRIP or IUCLC, processing is
identical to raw mode; handle this receiving mode as a special-case
of raw mode.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>