Fix default assignment for SPI chip select

[eberseth]

Problem

The SPI interface default chip select state is set to SPI_DEFAULT_SS which may override user defined pin map controls over GPIO pins. Normally the user will assign and manage their own chip select and not use the native SS pin from the MCU peripheral.

Solution

Replace references to the default MCU peripheral chip select pin, SS, with an invalid pin map entry, PIN_INVALID so that the user can specify GPIO.

Steps to Test

N/A

Example App

N/A

References

N/A

---

Completeness

• User is totes amazing for contributing!
• [x ] Contributor has signed CLA (Info here)
• [x ] Problem and Solution clearly stated
• Run unit/integration/application tests on device
• Added documentation
• Added to CHANGELOG.md after merging (add links to docs and issues)