net: openthread: Fix UART platform callback
net: openthread: Fix UART platform callback

OpenThread UART tx callback has been processed even if it was not
triggered by OpenThread otPlatUartSend function.

Signed-off-by: Lukasz Maciejonczyk <12a52743b42a5ad3dc4d3a6e4f8a76d0ad7c6806@nordicsemi.no>

Vudentz/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,galak/zephyr,nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,finikorg/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,nashif/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr,Vudentz/zephyr,galak/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,Vudentz/zephyr