SPI: Use the max clock frequency if unset (#327)

[MatthiasJentsch]

Description

Fixing "Getting 'System.ArgumentException' Error while calling 'SpiDevice.FromId'". The cause of this exception is the unset SPI clock frequency. Now if the frequency is unset we use the max frequency.

Motivation and Context

• Fixes Getting 'System.ArgumentException' Error while calling 'SpiDevice.FromId' Home#327

How Has This Been Tested?

Tested on ESP32 with the following C# code:
SpiDevice.FromId("SPI1", new SpiConnectionSettings(22));

Types of changes

• Improvement (non-breaking change that improves a feature, code or algorithm)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

Signed-off-by: Matthias Jentsch info@matthias-jentsch.de

[nfbot]

Hi @MatthiasJentsch,

I'm nanoFramework bot.
Thank you for your contribution!

A human will be reviewing it shortly. 😉

[AdrianSoundy]

Changes look ok