PNP: Set up pnp_debug via module and not via boot param.

Cleanup only, no functional change (pnp.debug can be enabled
and disabled at runtime, but that's not a real enhancement).

This one depends on another PNP cleanup patch:
PNP: Compile all pnp built-in stuff in one module namespace

Signed-off-by: Thomas Renninger <trenn@suse.de>
Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Len Brown <len.brown@intel.com>