drivers: serial: uart_nrfx_uarte: Fix interrupt driven TX only mode

When interrupt driven instance is using only TX then low power mode
is automatically enabled and then TXSTOPPED interrupt is disabled in
idle so uarte_nrfx_irq_tx_ready_complete() was returning wrong value.
Adding a dedicate flag which holds information is user enabled TX
interrupt.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>