The idea is to calibrate the LAPIC timer just once and only on boot,
given that [at present] the timer constants are global and shared
between all processors.

My primary motivation is to fix a panic that can happen when dynamically
switching to lapic timer.  The panic is caused by a recursion on
et_hw_mtx when printing the calibration results to console.  See the
review for the details of the panic.

Also, the code should become slightly simpler and easier to read.  The
previous code was racy too.  Multiple processors could start calibrating
the global constants concurrently, although that seems to have been
benign.

Reviewed by:	kib, mav, jhb
MFC after:	3 weeks
Differential Revision: https://reviews.freebsd.org/D15422