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@
Reviewed by: me (long ago)
MFC after: 22.57 seconds
Relnotes: yea
Differential Revision: https://reviews.freebsd.org/D16855