clockcalib: Fix an overflow bug

tc_counter_mask is an unsigned int and in the TSC timecounter is equal
to UINT_MAX, so the addition tc->tc_counter_mask + 1 can overflow to 0,
resulting in a hang during boot.

Fixes: c2705ceaeb09 ("x86: Speed up clock calibration")
Reviewed by: cperciva
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D33956