fix(cdc/ftdi): correct divisor selection in 1 MBaud to 2 MBaud range (IEC-534)

[spectrumjade]

fix(cdc/ftdi): correct divisor selection in 1 MBaud to 2 MBaud range

Description

Previously, baud rates between above 1 MBaud were short-circuited to a special case divisor (either 0 or 1), but had the wrong reported realized rate. In the case of baudrate >= 1000000, the selected divisor was 1, which programs the chip at 2 MBaud (per AN232B-05), not 1 MBaud as claimed in the comment and set as baudrate_real. In addition, rates that the general divisor path could have handled (notably including 1.5 MBaud, which uses even divisor of 2) were being silently coerced to 2 MBaud.

This PR corrects the two special case divisors and allows baud rates below 2 MBaud to be handled by the general divisor path. This allows 1.5 MBaud to work properly.

Testing

Confirmed that an FT232R, which previously was not successfully able to function at a requested 1.5 MBaud, is now working properly at 1.5 MBaud.

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Checklist

Before submitting a Pull Request, please ensure the following:

• 🚨 This PR does not introduce breaking changes.
• All CI checks (GH Actions) pass.
• Documentation is updated as needed.
• Tests are updated or added as necessary.
• Code is well-commented, especially in complex areas.
• Git history is clean — commits are squashed to the minimum necessary.

[tore-espressif]

@spectrumjade thank you for the contribution!

I'll have to check why we did not enable 1.5Mbit. IIRC, the chips that I tested with did not support this baudrate. I'll double check so we can merge this PR, thanks!