SPI.beginTransaction allows SPISettings faster than 8 MHz

[PaulStoffregen]

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.