power: replace unsigned -1 with unsigned maximum

power: replace unsigned -1 with unsigned maximum

Use UINT64_MAX instead of -1ULL.

Some compilers generate a warning when applying a '-' to
an unsigned literal so avoid this by initializing with
unsigned preprocessor definition.

Signed-off-by: Tyler Retzlaff <roretzla@linux.microsoft.com>
Acked-by: Morten Brørup <mb@smartsharesystems.com>
Acked-by: Anatoly Burakov <anatoly.burakov@intel.com>