powerpc: Fallout from sysdev API changes

A struct sysdev_attribute * parameter was added to the show routine by
commit 4a0b2b4dbe1335b8b9886ba3dc85a145d5d938ed "sysdev: Pass the
attribute to the low level sysdev show/store function".

This eliminates a warning:

arch/powerpc/kernel/sysfs.c:538: warning: initialization from incompatible pointer type

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>