decodetree: Use Python3 floor division operator

decodetree: Use Python3 floor division operator

This script started using Python2, where the 'classic' division
operator returns the floor result. In commit 3d004a371 we started
to use Python3, where the division operator returns the float
result ('true division').
To keep the same behavior, use the 'floor division' operator "//"
which returns the floor result.

Fixes: 3d004a371
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20200330121345.14665-1-f4bug@amsat.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>