Input: libps2 - relax command byte ACK handling

When we probe PS/2 devices we first issue "Get ID" command and only if we
receive what we consider a valid keyboard or mouse ID we disable the device
and continue with protocol detection. That means that the device may be
transmitting motion or keystroke data, while we expect ACK response.

Instead of signaling failure if we see anything but ACK/NAK let's ignore
"garbage" response until we see ACK for the command byte (first byte). The
checks for subsequent ACKs of command parameters will continue be strict.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>