coding guidelines: comply with MISRA C:2012 Rule 13.3

In particular:

- avoided ++/-- on volatile

- avoided to have more than one ++/-- in for third clause

- moved ++/-- before or after the value use

- avoided pointless ++/--

- replaced consecutive ++ with a single +=

Signed-off-by: Abramo Bagnara <abramo.bagnara@bugseng.com>