asr(4): Remove a case that is not a member of the enum being tested.

Apparently xpt_opcode can wind up being REPORT_LUNS here. gcc47 warns
about it because REPORT_LUNS is not an enum xpt_opcode member.

FreeBSD just casted this away. Instead of doing that, remove it and
let the default case handle it (which has the same code as XPT_ABORT).

While here, fix a typo in a comment.