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
Differential Revision: https://reviews.freebsd.org/D33696

(cherry picked from commit 698727d637341f2306138ca0ea181444de3df479)