nrf_usbd_common: Do not trigger DMA in low power mode

DMA transfers do not execute when the USBD peripheral is in Low Power
mode. Make sure that there is no DMA active transfer when entering Low
Power mode and that new DMA transfers are not started when in Low Power
mode because the transfer won't ever finish.

Signed-off-by: Tomasz Moń <tomasz.mon@nordicsemi.no>