[POWERPC] Fix crash in init_ipic_sysfs on efika

The global primary_ipic in arch/powerpc/sysdev/ipic.c can remain NULL
if ipic_init() fails, which will happen on machines that don't have an
ipic interrupt controller.  init_ipic_sysfs() will crash in that case.

Acked-by: Grant Likely <grant.likely@secretlab.ca>

Signed-off-by: Paul Mackerras <paulus@samba.org>