powerpc/maple: Fix comparing pointer to 0

Fixes coccicheck warning:
  arch/powerpc/platforms/maple/setup.c:232:15-16:
	WARNING comparing pointer to 0

Compare pointer-typed values to NULL rather than 0.

Signed-off-by: Chen Zhou <chenzhou10@huawei.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200121013153.9937-1-chenzhou10@huawei.com