ipmi: Remove ACPI SPMI probing from the system interface driver

The IPMI spec states:

  The purpose of the SPMI Table is to provide a mechanism that can
  be used by the OSPM (an ACPI term for “OS Operating System-directed
  configuration and Power Management” essentially meaning an ACPI-aware
  OS or OS loader) very early in the boot process, e.g., before the
  ability to execute ACPI control methods in the OS is available.

When we are probing IPMI in Linux, ACPI control methods are available,
so we shouldn't be probing using SPMI.  It could cause some confusion
during the probing process.

Signed-off-by: Corey Minyard <cminyard@mvista.com>