Fix: bashate E043 warnings

Arithmetic compound has inconsistent return semantics

((expr)) has a trap that if expr is 0, the return value is 1.
This will trigger a failure with "set -e" and can be very confusing.
It is good defensive programming to avoid this with explicit assignment.

Signed-off-by: Guillaume Lambert <guillaume.lambert@orange.com>
Change-Id: I412bd8872560c90c28642ed5433eab9b2a29a5e8