cxgbe: Compute timestamps via sbintime_t.

This uses fixed-point math already used elsewhere in the kernel for
sub-second time values. To avoid overflows this does require updating
the calibration once a second rather than once every 30 seconds. Note
that the cxgbe driver already queries multiple registers once a second
for the statistics timers. This version also uses fewer instructions
with no branches (for the math portion) in the per-packet fast path.

Reviewed by: np
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D36663