powerpc: Pass the right cpu_spec to ->setup_cpu() on 64-bit

When calling setup_cpu() on 64-bit, we pass a pointer to the
cputable entry we have found. This used to be fine when cur_cpu_spec
was a pointer to that entry, but nowadays, we copy the entry into
a separate variable, and we do so before we call the setup_cpu()
callback. That means that any attempt by that callback at patching
the CPU table entry (to adjust CPU features for example) will patch
the wrong table.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>