cam: enable kern.cam.ada.enable_uma_ccbs by default

This makes the ada(4) driver use UMA for its CCBs. While it's
da(4) counterpart needs some more testing, this one seems to be
safe now.

Please let me know via email if you notice any suspicious kernel
messages,

Reviewed By: imp
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D30567