x86/ACPI/cstate: Make APCI C1 FFH MWAIT C-state description vendor-neutral

Commit 5209654a (x86/ACPI/cstate: Allow ACPI C1 FFH MWAIT use on
AMD systems) forgot to update the ACPI C1 idle state description and
tools like turbostat display "ACPI FFH INTEL MWAIT 0x0" which is
quite confusing on an AMD system.

Drop the "INTEL" part from the ACPI C1 FFH MWAIT C-state description
to avoid confusion.

Fixes: 5209654a

 (x86/ACPI/cstate: Allow ACPI C1 FFH MWAIT use on AMD systems)
Signed-off-by: Prarit Bhargava <prarit@redhat.com>
[ rjw: Subject & changelog ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>