PM / Domains: check for negative return from of_count_phandle_with_args()

The return from of_count_phandle_with_args can be negative, so we
should avoid kcalloc of a negative count of genpd_power_stat structs
by sanity checking if count is zero or less.

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Kevin Hilman <khilman@baylibre.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>