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

[jhb]

Summary

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.

[gallatin]

Neat. I was not aware of seqc(9) until now!