drivers: serial: nrfx_uarte: Fix low power mode for interrupt driven API

Low power mode for non-asynchronous API case is only available when
RX is not used (RX pin is not defined). In that case TX starting
function was using uarte_enable_locked() which tracks if UARTE is
used by TX or RX and TXSTOPPED interrupt was disabling UARTE
unconditionally. Because of that following attempt to TX start
was assuming that UARTE is already enabled when it was disabled.
Fixing it by using uarte_disable_locked function in TXSTOPPED
handling.

Code reordering was required to make uarte_disable_locked()
available earlier.

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