n_tty: Split n_tty_receive_char()

Factor 'special' per-char processing into standalone fn,
n_tty_receive_char_special(), which handles processing for chars
marked in the char_map.

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