target/arm: Fix ID_MMFR4 value on AArch64 'max' CPU

target/arm: Fix ID_MMFR4 value on AArch64 'max' CPU

In commit 41a4bf1feab098da4cd the added code to set the CNP
field in ID_MMFR4 for the AArch64 'max' CPU had a typo
where it used the wrong variable name, resulting in ID_MMFR4
fields AC2, XNX and LSM being wrong. Fix the typo.

Fixes: 41a4bf1feab098da4cd
Reported-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Reviewed-by: Laurent Desnogues <laurent.desnogues@gmail.com>
Message-id: 20200422124501.28015-1-peter.maydell@linaro.org