camcontrol: Simplfiy 12-byte retry logic in defects command

We always start out using the 10-byte version of READ DEFECT DATA, and
then switch to 12-byte when necessary due to errors or data length
requirements. We always need to get the length again when we do this,
and we're always going to be using 12-byte commands from that point
forward. Simplify the logic a bit based on this observation.

Sponsored by: Netflix
Reviewed by: mav
Differential Revision: https://reviews.freebsd.org/D40522