xen/privcmd: return -ENOTTY for unimplemented IOCTLs

The code sets the default return code to -ENOSYS but then overrides this
to -EINVAL in the switch() statement's default case, which is clearly
silly.

This patch removes the override and sets the default return code to
-ENOTTY, which is the conventional return for an unimplemented ioctl.

Signed-off-by: Paul Durrant <paul.durrant@citrix.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>