driver:tla2518: Fixed read function and SPI communication

The read function wasn't working, because of some wrong assumption about the readback of values in the initial implementation. Also, the SPI clock of 60 MHz led to errors on the bus. The 25 MHz clock locks successfully on the PLL and also seems to be stable.

Note: The separate setup of the channel has been removed, because the idea of the read function is to select the channel, wait and then read the result.

BCS 746-001106