Update SPI to work with STM32

[peterpolidoro]

Hi Peter,

I want to use your AD57x4 library with a MapleMini board (and with AD5732AREZ),
but I get an error in the Arduino IDE when compiling it: AD57X4R.cpp:474:68:
error: invalid conversion from 'uint8_t {aka unsigned char}' to 'BitOrder'
[-fpermissive]SPI.beginTransaction(SPISettings(SPI_CLOCK,SPI_BIT_ORDER,SPI_MODE));

I assume the STM32F103 uses a different SPI implementation. What do you suggest,
what should be changed?

Thanks in advance, Donat.