Changed SPI speed to configurable

[ken551]

Hello.

I modified SPI speed from the fixed value (10M) to the configurable with the SPI_SPEED definition in mcp_can.h .

In my environment, SPI with the speed of 10M didn't work but with slower speed (e.g. 1M, 4M) worked. This PR makes it easier to change SPI speed in such a situation.

Hope this helps.