drivers/uart: stm32: don't call k_yield on poll_out

Some tests like:
    tests/kernel/sched/metairq/kernel.scheduler.metairq
    tests/kernel/profiling/profiling_api/kernel.common.profiling
    tests/kernel/sched/schedule_api/kernel.scheduler
    tests/kernel/sched/schedule_api/kernel.scheduler.multiq
    tests/kernel/profiling/profiling_api/kernel.common.profiling
    tests/kernel/workq/work_queue/kernel.workqueue

don't support that the current thread change when writing a message with
printk (which uses poll_out). So, we remove the call to k_yield which is
useful only for optimizing cpu usage by forcing a thread change if the
usart send stack is full.

Signed-off-by: Julien D'ascenzio <julien.dascenzio@paratronic.fr>