[SPI] 16 bit transactions lead to comunication issues

While debugging ubxgps and sx1278 spi drivers we found out that high SPI dividers lead to issues in the communication. This was caused (still with unknown causes) by a subsequent operation of 8 bits and an operation of 16 bits. The effects of this bug is that the communication works in a range of DIVs. With too high DIVs the communication was corrupted: in ubxgps-spi, after reading two uint8_t, the subsequent uint16_t reads were corrupted (the first one read the last uint8_t and the first new byte; the second one jumped 2 bytes and read the following 2 bytes).