PNP: Hide pnp_bus_type from the non-PNP code

The pnp_bus_type is defined only when CONFIG_PNP=y, while being
not guarded by ifdeffery in the header. Moreover, it's not used
outside of the PNP code. Move it to the internal header to make
sure no-one will try to (ab)use it.

Signed-off-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>