Implement requestSPIBusRelease()

The function in the title will have the purpose of requesting the SPI bus release on the part of the given device. Once the corresponding command is sent to the target MCP2210 device, the SPI bus will be released or not, depending on whether an SPI transfer is ongoing. The function accepts one argument, corresponding to the value that the SPI Bus Release ACK should assume (if GP7 is assigned to that dedicated function).