leds: powernv: removing NULL check

No need to explicitly check for pointer to be null,
of_prop_next_string anyways returns NULL, if passed pointer is NULL
and hence loop will continue.

Signed-off-by: Saurabh Sengar <saurabh.truth@gmail.com>
Acked-by: Vasant Hegde <hegdevasant@linux.vnet.ibm.com>
Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>