cxgbe: Use secq(9) to manage the timestamp generations.

This is mostly cosmetic, but it also doesn't leave a gap of time where
no structures are valid. Instead, we permit the ISR to continue to
use the previous structure if the write to update cal_current isn't
yet visible.

Reviewed by: gallatin
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D36669