Address linter issues

Newer versions of the linter flags issues with parentheis in
expressionss, as well as use of `size_t` without inclusion of `stddef.h`

Signed-off-by: Emil Ohlsson <emil.ohlsson@arm.com>

Reviewed-by: Emil Ohlsson <emil.ohlsson@arm.com>
Approved-by: Jakub Sujak <jakub.sujak@arm.com>