tty: incorrect test of echo_buf() result for ECHO_OP_START

test echo_buf() result for ECHO_OP_START

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Acked-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>