arch: Fix the code that computes MaxMiscDestReg.

arch: Fix the code that computes MaxMiscDestReg.

This code was simplified a little while ago, and the wrong variable name
was used in that computation accidentally. Fortunately the "wrong" value
would be too large, and so nothing bad would happen except a pair of
arrays would be overly large in the O3 instruction class.

Change-Id: I9694f1a8c79a62a172ef63bdd2f98fa0ace06acd
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/38383
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>