Fix flow control setting issue, fix #316

[mwcmwc12]

Flow control for serial device cannot be set to none as 0 or ControlFlow.none is treated as an invalid attribute. Changed to "0 <="

• [x] Closes # 316
• [x] Executed black . && isort -c . && flake8 with no errors
• [ ] The change is fully covered by automated unit tests
• [ ] Documented in docs/ as appropriate
• [x] Added an entry to the CHANGES file

[mwcmwc12]

Accidentally closed comment, reopening.