serial: add extended Linux baud rate support

[WangHanChi]

Pull request

Choose Correct

• bug
• feature

Describe the pull request

This PR extends Linux serial_setup() to support additional standard baud rates:

• 19200
• 230400
• 460800
• 921600
  Unsupported baud rates now gracefully fallback to 57600 instead of failing.
  This improves compatibility with higher-speed serial devices commonly used in embedded and industrial systems.

To Reproduce

1. Open a serial device with a baud rate of 230400, 460800, or 921600.
2. Call serial_setup() with the new baud rate.

Expected behavior

• The serial device should initialize successfully at the requested baud rate.
• For unsupported baud rates, it will fallback to 57600 instead of erroring out.

Screenshots

N/A

Desktop (please complete the following information):

• OS : ubuntu 22.04
• eRPC Version : v1.14.0

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• I checked if other PR isn't solving this issue.
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• PR code is tested.
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Additional context