n_tty: Factor 'real raw' receive_buf into standalone fn

Convert to modal receive_buf() processing; factor real_raw
receive_buf() into n_tty_receive_buf_real_raw().

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