Use the explicit expanded form of cmp.

Clang apparently requires the explicit form of this instruction, and rejects
uses which ignore the optional cmpD register. This was the only use of the
shorthand form of the instruction, so just fix it up to match the others.

PR: kern/215681
Submitted by: Mark Millard
Reported by: Mark Millard <markmi _AT_ dsl-only.net>
MFC after: 2 weeks