ipmi: convert tracking of the ACPI device pointer to a PNP device

Convert PNP patch (git 9e368fa0) to
maintain a pointer to a PNP device, 'pnp_dev', instead of the ACPI device,
'acpi_dev', that is currently being tracked with PNP based IPMI device
discovery.

Signed-off-by: Myron Stowe <myron.stowe@hp.com>
Acked-by: Zhao Yakui <yakui.zhao@intel.com>
Acked-by: Corey Minyard <minyard@acm.org>
Cc: Len Brown <lenb@kernel.org>
Cc: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>