This reverts commit a9faac7218e86f9c44a34dabec1d509c434bdc36.
The changed rounding behavior sets the SPI baud rate to 4MHz, which is a suitable rate for the AT86RF212B.
This board is driven by a 16MHz clock source. In this case, the baud rate generator is able to clock the SPI interface with 4MHz or 8MHz only.
Due to integer division and rounding, the interface is actually driven with 8MHz, which is too fast for the AT86RF212.
