[spi][stm32] updated handling of different transaction lengths, now tx buffer sizes are independent of rx buffer sizes, uses a second dummy dma transmit transfer triggered of the first transmit dma ISR when 0 < tx_len < rx_len
[spi] updated some comments
[peripherals] adxl345: enable measurement mode AFTER other configuration is done
[peripherals] adxl345_spi: set input/output length of spi transaction to actual lenghts