powerpc/powernv: add NULL check after kzalloc

Fixes coccicheck warning:

./arch/powerpc/platforms/powernv/opal.c:813:1-5:
	alloc with no test, possible model on line 814

Add NULL check after kzalloc.

Signed-off-by: Chen Zhou <chenzhou10@huawei.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200509020838.121660-1-chenzhou10@huawei.com