drivers: serial: nrfx_uarte: Fix runtime PM for interrupt driven API

TX part of interrupt driven API was not calling PM device runtime
API. Additionally, when txstopped occurred after poll out it was
not handled correctly.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>
(cherry picked from commit 13ed9e42)