drivers: serial: Fix race condition in nRF5 UART TX
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 <f23d94b2a735b6193d3882ae31308d2e41e8282b@exati.com.br>

kraj/zephyr,galak/zephyr,zephyriot/zephyr,galak/zephyr,explora26/zephyr,Vudentz/zephyr,mbolivar/zephyr,Vudentz/zephyr,mbolivar/zephyr,mbolivar/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,zephyriot/zephyr,kraj/zephyr,nashif/zephyr,explora26/zephyr,finikorg/zephyr,punitvara/zephyr,punitvara/zephyr,Vudentz/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,zephyriot/zephyr,explora26/zephyr,galak/zephyr,ldts/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyriot/zephyr,Vudentz/zephyr,zephyriot/zephyr,ldts/zephyr,punitvara/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,kraj/zephyr,explora26/zephyr,mbolivar/zephyr,Vudentz/zephyr,finikorg/zephyr,Vudentz/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,galak/zephyr,ldts/zephyr,finikorg/zephyr,nashif/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,mbolivar/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr