Fix variable name: freq_khz -> freq

An earlier version of this code computed the TSC frequency in kHz.
When the code was changed to compute the frequency more accurately,
the variable name was not updated.

Reviewed by: markj
Fixes: 22875f88799e x86: Implement deferred TSC calibration
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D33696