dm flakey: return -EINVAL on interval bounds error in flakey_ctr()

Fix to return error code -EINVAL instead of 0, as is done elsewhere in
this function.

Fixes: e80d1c80 ("dm: do not override error code returned from dm_get_device()")
Cc: stable@vger.kernel.org # 4.3+
Signed-off-by: Wei Yongjun <weiyj.lk@gmail.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>