xinput: rep SetDeviceMode.status: altenum to enum

xinput: rep SetDeviceMode.status: altenum to enum

Field "status" of the SetDeviceMode-reply
only allows values from enum GrabStatus.

Therefore it has an "enum" and not an "altenum".

Also: Add comment about valid values of field "status".

spec:
http://cgit.freedesktop.org/xorg/proto/inputproto/tree/specs/XIproto.txt#n601

note:
The following spec also mentions value "3 + first_error":
http://cgit.freedesktop.org/xorg/lib/libXi/tree/specs/encoding.xml#n891

This is probably an error in this old spec because:
* the X-Server code does not generate that value.
  In case of error-codes, it does the following:
  instead of sending the reply it returns
  rep.status containing the error-code as returnvalue
  of the function ProcXSetDeviceMode.
  This will probably cause the sending of an XError.
  code:
    http://cgit.freedesktop.org/xorg/xserver/tree/Xi/setmode.c#n86

* the new spec does not mention that value:
  http://cgit.freedesktop.org/xorg/proto/inputproto/tree/specs/XIproto.txt#n601