PNP: update pnp.debug usage (needs value on command line)

Commit cdefba03 changed pnp.debug from a boot param to a module param,
which means it needs a value when used on the command line.

CC: Thomas Renninger <trenn@suse.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Signed-off-by: Len Brown <len.brown@intel.com>