modbus: client: change 'EXNO' to '-EIO'

[aatoof1]

Fix error in Modbus client write response validation.

See discussion here

[github-actions]

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud