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

This makes the da(4) driver use UMA for its CCBs by default,
like ada(4) already does. 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/D31257