python: Fix toBool converter

python: Fix toBool converter

It was using an undefined variable (result) which was mistakenly left
there after its latest refactor

Change-Id: I50bb9b1e7793045556a29306faea5f455b29819d
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/37755
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>