MFC r340426 (by cem): amdtemp(4): Fix temperature reporting on AMD 2990WX

Update the AMD family 17h temperature reporting based on AMD Tech Doc 56255
OSRR, section 4.2.1.

For CPUS w/CUR_TEMP_RANGE_SEL set, scale the reported temperature into the
range -49..206; i.e., subtract 49°C.

Submitted by: gallatin@
Reported by: bcran@