shell: fix polling TXDONE signal by multiple threads

This patch fixes the issue that can cause a deadlock in shell.
When two threads simultaneously poll the TXDONE signal, only one
of them will receive it, leaving second one stalled.
The problem was that shell's context contains k_poll_event objects that
were polled by multiple threads. Polling it overwrites the poller field
that was set by previous thread.
Instead, the k_poll_event object must be created on the stack by every
thread that wants to poll the TXDONE signal.
This makes sure that no thread will be left waiting for this signal
forever.

Signed-off-by: Michał Barnaś <mb@semihalf.com>