Fix checks in LUSetNodeParms for the master node

There was a check already in the LU for the master node, however is
wasn't correct. This patch disallows any role changes on the master node
via LUSetNodeParms (and as this LU can't change anything else, it
practically prevents it from touching the master node).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>