SPI.beginTransaction allows SPISettings faster than 8 MHz

The SPI port on Arduino 101 does not properly receive MISO input when the clock is configured faster than 8 MHz. However, SPI.beginTransaction() does not properly limit the speed to 8 MHz when a program attempts to configure higher clock speeds.

I added a workaround in my work on the Ethernet library.
https://github.com/PaulStoffregen/Ethernet/commit/d7155e132c15823c7314e545bf2ae39ee1c27a75

I understand Arduino 101 is discontinued and nobody is likely to ever update the SPI library or other code. But created this issue to document the problem. Hopefully it'll help anyone else who tries to support SPI-based libs on Arduino 101.

Start with SPI.beginTransaction() and SPISettings, then inspect the Ethernet workaround commit linked in the report. Reproduce the Arduino 101 behavior above 8 MHz and trace where the requested clock is applied. Done means unsupported speeds are limited to 8 MHz and the behavior has a regression check.

Written by the indexing model from the issue text.