[media] stv090x: do not unlock unheld mutex in stv090x_sleep()

goto err and goto err_gateoff before mutex_lock(&state->internal->demod_lock)
lead to unlock of unheld mutex in stv090x_sleep().
Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Cc: Manu Abraham <abraham.manu@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>