power-domain: Fix use of uninitialized value in dev_power_domain_ctrl

If dev_count_phandle_with_args returns 0 or another error, then pd will never
have been initialized by power_domain_get_by_index. Avoid comparing against
pd.dev in this situation.

Fixes: 3e4fcfa4 ("power-domain: fix hang in endless loop on i.MX8")
Signed-off-by: Sean Anderson <seanga2@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Jaehoon Chung <jh80.chung@samsung.com>