drivers: serial: Fix race condition in nRF5 UART TX

Fix a somewhat rare race condition when the thread gets
preempted in the middle of sending a byte through UART.

If the other thread also sends another byte through UART
and "consumes" the EVENTS_TXDRDY value, the first thread
will get stuck in the while loop forever.

By moving the reset to the function start, we guarantee that
the baseline state of EVENTS_TXRDY is 1. Therefore, the first
thread will continue normally when it executes again.

Signed-off-by: Thiago Silveira <thiago@exati.com.br>