serial: uart_nrfx_uarte: poll & async TX infinite hang

[JordanYates]

Describe the bug
When using uarte_nrfx_poll_out and uarte_nrfx_tx from different threads in the same application,
the poll_out thread can block forever, and all calls to uarte_nrfx_tx will result in failure.

This is a continuation of issue #22264

To Reproduce

1. Setup a nrf52840dk to use the UARTE driver.
2. Add 2 threads to any sample, one of which calls printk in a loop, the other which calls uart_tx
3. Have the poll thread preempt the uart_tx thread
4. Poll out will never return, resulting in infinite hang.

I have pushed a commit based on Zephyr 2.3rc1 that demonstrates this issue on the blinky sample application.
https://github.com/JordanYates/zephyr/commit/87d296baca519294d28a8394f40022a42ef82991

Expected behavior

Both the poll_out and uart_tx threads should forever print out their respective messages.
The provided reproduction will instead print out 3 "I poll" messages before stopping forever.

Impact
Major annoyance

Environment (please complete the following information):

• Zephyr 2.3rc1

[JordanYates]

And of course I then realize its a thread priority issue, nothing to do with the driver itself.
Apologies.