nrf: new_txonly_nosck() does not work with most SPIM instances

[timokroeger]

While working with WS2811 LEDs I found that

embassy/embassy-nrf/src/spim.rs

Line 151 in c65b6db

pub fn new_txonly_nosck(

does not actually work. This is a known issue as described here: zephyrproject-rtos/zephyr#57147 (comment)

The above comment indicates that nosck is possible with SPIM3 instance on nrf52840 devices. But even there I found that the MOSI pin is only driven during the transaction and goes High-Z otherwise.