- Added SpiDataSize enum to allow for non 8-bit data transfers
- Moved cr2 write after cr1 write, based on section 39.5.7 in
the rm0440 manual
- Added is_busy function to check if the SPI bus is busy
- Added unchecked_send_u8 and unchecked_send_u16
that writes to the data register but doesn't check the status
 register. It does check and wait for the busy bit to be low; without
 this check the data was not transmitting correctly.