pmcr: Fix pstate setting on Power8

Fix p-state setting on Power8 by removing the accidental double-indirection of
the pstate_ids table.

The pstate_ids table comes from the OF property "ibm,pstate-ids." On Power9,
the values happen to be identical to the indices, so the extra indirection was
harmless. On Power8, the values were out of the range [0, npstates], so
pmcr_set() would fail the spec[0] range check with EINVAL.

While here, include both the value and index in the driver-specific register
array as spec[0] and spec[1] respectively. They're redundant, but relatively
harmless, and it may aid debugging.

While here, fix the range check to exclude the index npstates, which is one
past the last valid index.

PR: 233693
Reported and tested by: sbruno
Reviewed by: jhibbits