arcmsr(4): don't bzero newly allocated ccb; xpt_alloc_ccb() already does that.

Reviewed by: delphij, imp, ching2048@areca.com.tw
Tested by: ching2048@areca.com.tw
MFC after: 2 weeks
Sponsored by: NetApp, Inc.
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D26840