n_tty: Factor ISTRIP and IUCLC receive_buf into separate fn

Convert to modal receive_buf processing; factor char receive
processing for unusual termios settings out of normal per-char
i/o path.

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