cpuidle/powernv: cleanup cpuidle-powernv.c

cpuidle/powernv: cleanup cpuidle-powernv.c

- Use stack instead of kzalloc'ed memory for variables while probing
   device tree for idle states.
 - Set cap for number of idle states that can be added to
   cpuidle_state_table
 - Minor change in way we check of_property_read_u32_array for error
   for sake of consistency
 - Drop unnecessary "&" while assigning function pointer

Cc: Rafael J. Wysocki <[email protected]>
Cc: Daniel Lezcano <[email protected]>
Cc: [email protected]
Signed-off-by: Shreyas B. Prabhu <[email protected]>
Signed-off-by: Michael Ellerman <[email protected]>