drivers: uart_nrfx_uart: Fix disabling of TX IRQ

The STOPTX task cannot be triggered directly in the function that
disables TX interrupt because this task stops the UART transmitter
immediately, even if it is in the middle of shifting out a byte.
Instead, this task needs to be triggered in the interrupt handler,
when the end of transmission is signaled.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>