The devices are allowed to respond to either command byte or command
parameter with a NAK (0xfe), and the host is supposed to resend the
"correct" byte. The device then will either respond with ACK or ERR (0xfc).
Let's teach libps2 to handle the NAK responses properly, so that individual
drivers do not need to handle them.
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>