style: subsys: adjust `return` usage in `void functions`

For code clarity, this commit adjusts the use of `return` statements
in functions with a void return type as follows:
- Transform `return foo();` into separate statements:
  `foo();`
  `return;`
- Remove unnecessary `return` statements when
  they don't affect control flow.

Signed-off-by: Pisit Sawangvonganan <pisit@ndrsolution.com>