drivers: serial: nrfx_uarte: Fix power management

Power consumption was still high after putting uarte device into off
state. It was caused by ENDRX interrupt that was triggered after
calling STOPRX. ENDRX event was called with 0 amount but interrupt
got triggered and fifo_read was starting RX again.

Added disabling RX interrupt before disabling UARTE and reenabling at
device activation.

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