[CPUFREQ] powernow-k8: Fix indexing issue

The driver uses the pstate number from the status register as index in
its table of ACPI pstates (powernow_table). This is wrong as this is
not a 1-to-1 mapping.

For example we can have _PSS information to just utilize Pstate 0 and
Pstate 4, ie.

  powernow-k8: Core Performance Boosting: on.
  powernow-k8:    0 : pstate 0 (2200 MHz)
  powernow-k8:    1 : pstate 4 (1400 MHz)

In this example the driver's powernow_table has just 2 entries. Using
the pstate number (4) as index into this table is just plain wrong.

Signed-off-by: Andreas Herrmann <andreas.herrmann3@amd.com>
Signed-off-by: Dave Jones <davej@redhat.com>