ACPI: thermal: Get rid of a dummy local variable

The second argument of acpi_bus_update_power() can be NULL, so drop the
power_state dummy local variable in acpi_thermal_resume() used just for
avoiding passing NULL as the second argument to that function.

No intentional functional impact.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Link: https://patch.msgid.link/13851842.uLZWGnKmhe@rafael.j.wysocki