target-arm: Correct check for non-EL3

This fixes a compile warning from clang 3.5 (the assertion
could never fire).

Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Message-id: 1432881807-18164-2-git-send-email-edgar.iglesias@gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: John Snow <jsnow@redhat.com>
[PMM: added note in commit message that this is fixing a build warning]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
