powerpc: pcibios_enable_device_hook: return bool rather than int

pcibios_enable_device_hook returned an int. Every implementation
returned either -EINVAL or 0. The return value wasn't propagated by
the caller: any non-zero return value caused pcibios_enable_device
to return -EINVAL itself. Therefore, make the hook return a bool.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>