powerpc/ps3: Fix sys-manager-core sparse warnings

Fixes warnings like these:

  drivers/ps3/sys-manager-core.c: error: symbol 'ps3_sys_manager_power_off' redeclared with different type

Signed-off-by: Geoff Levand <geoff@infradead.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>