[netX10/50/51] - SPI transfer fails for high frequencies (>= 5MHz) if the second spi bus works with frequencies lower than 5 MHz

Due to an corrupt ISR of the spi driver, interrupt and polling mode cannot be used at the same time on the two available spi buses. As the mode is set according to the operation frequency, spi transfers with high frequencies (>= 5MHz) will fail, if the other bus uses frequencies below 5 MHz at the same time. This bug is a side effect of RCX-494, where SPI transfers in polling mode were introduced to boost performance.
Affected chips: netX10/50/51/52