PNP: turn on -DDEBUG when CONFIG_PNP_DEBUG is set

Turn on -DDEBUG in CFLAGS when CONFIG_PNP_DEBUG=y.  This makes
dev_dbg() do what you expect.

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Acked-by: Rene Herman <rene.herman@gmail.com>
Signed-off-by: Len Brown <len.brown@intel.com>