Fix _cpu_freq_freqspec_num() to return the highest frequency when greater value requested.

Code would return NO_VAL if the requested frequency was greater than
the highest available.

While here improve the errors printed to the slurmstepd log location,
and change the initial check against nfreq to test for zero. (Rather
than (uint8_t) NO_VAL which it could never be set to.)

Bug 3335.