drivers:platform:stm32:Fixes to PWM driver

[jansunil]

The PWM polarity was being hardcoded to TIM_OCPOLARITY_HIGH, despite there being a no-OS structure member for polarity

Fixes: 9264d89 ("stm32 pwm support")

Pull Request Description

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PR Type

• [x] Bug fix (change that fixes an issue)
• [ ] New feature (change that adds new functionality)
• [ ] Breaking change (has dependencies in other repos or will cause CI to fail)

PR Checklist

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