[stm32][spi] some renaming/cleanup, hopefully fix 16bit transfer as well...
[peripherals] adxl345_spi stuff
[spi][stm32] added better handling of different length cases between tx and rx buffers, use dummy buffer for tx 0 length transfer, now only need to make sure the tx buffer is at least as long as the rx buffer in the case that 0 < output_length < input_length, other cases should be handled fine regardless of buffer lengths
[spi] added comment about buffer length requirements for stm32