mca: use time_uptime instead of ticks for CMCI throttling

This solves several problems.
First of all, cmc_throttle is specified in seconds and there was no
conversion between ticks and seconds when they were mixed together.
Second, we avoid potential problems with ticks wrapping around.

Resolution of time_uptime should be sufficient for the throttling
purposes.

Discussed with: jhb
MFC after: 12 days