powerpc/scom: Return NULL instead of 0

Function scom_map_device() returns data type 'scom_map_t', which is a
typedef for 'void *'. This functions is currently returning NULL and zero,
which causes the following warning by 'sparse':

	arch/powerpc/sysdev/scom.c:63:24: warning: Using plain integer as NULL pointer
	arch/powerpc/sysdev/scom.c:86:24: warning: Using plain integer as NULL pointer

This patch simply replaces zero by NULL.

Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>