PNP: add pnp_eisa_id_to_string()

Converting the EISA ID to a string is messy and error-prone, and
we might as well use the same code for ISAPNP and PNPBIOS.

PNPACPI uses the conversion done by the ACPI core with
acpi_ex_eisa_id_to_string().

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>