cpucontrol: Be more strict with input validation

Avoid truncating 32-bit values. This would have saved me a bit of time
when I was looking at a cpuid leaf on my system and typed 0x80000001f
instead of 0x8000001f.

Reviewed by: kib
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D54919