Running an MOSI only SPI (setting `sck` to `None`) causes SPI to hang

[korken89]

Hi,

I was testing using the SPI to drive a smart LED where I only needed the MOSI pin, so I set the sck pin to None.
However if one does this the SPI gets stuck in nrf_hal_common::spim::Spim<T>::do_spi_dma_transfer.
Adding back a clock pin does make it work again.

I'm not sure why this happens, and I can't find a reference in the datasheet to this behavior.
Does anyone know how to get this to work without having to have an SCK pin defined?