misc: zynq-xadc: Fix off-by-one

This bounds check was off-by-one. Fix.

Reported-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Peter Crosthwaite <crosthwaite.peter@gmail.com>
Message-id: 1453101737-11255-1-git-send-email-crosthwaite.peter@gmail.com
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
