PM / clk: ensure we don't allocate a -ve size of count clks

It is entirely possible for of_count_phandle_wit_args to
return a -ve error return value so we need to check for this
otherwise we end up allocating a negative number of clk objects.

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