mca_log: add missing breaks when handling thresholds

Per the "Intel® 64 and IA-32 Architectures Software Developer’s Manual
Combined Volumes: Vol. 3B 17-7", the Green and Yellow threshold
indicators are mutually exclusive. Add the missing break statements so
they're treated that way.

MFC after: 1 week
Reported by: Coverity
Differential Revision: https://reviews.freebsd.org/D48722