subprocess: Uniformize read access to `proc->ret`

[kazarmy]

Your checklist for this pull request

• [X] I've read the guidelines for contributing to this repository
• [X] I made sure to follow the project's coding style
• [ ] I've documented or updated the documentation of every function and struct this PR changes. If not so I've explained why.
• [ ] I've added tests that prove my fix is effective or that my feature works (if possible)
• [ ] I've updated the rizin book with the relevant information (if needed)

Detailed description

This pr just uniformizes read access to proc->ret in subprocess.c so that it all goes through rz_subprocess_ret().

Test plan

All builds are green.

Closing issues

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