PNP: add pnp_alloc_dev()

Add pnp_alloc_dev() to allocate a struct pnp_dev and fill in the
protocol, instance number, and initial PNP ID.  Now it is always
valid to use dev_printk() on any pnp_dev pointer.

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>