drivers: nrf: Fix RX timeout and ENDRX ISR handling.

Ensure that two routines won't interfere with eachother.
In current situation there is possibility that ENDRX will be called
during rx_timeout routine or vice-versa which will result in wrong
offset and length passed to user.

Signed-off-by: Mieszko Mierunski <mieszko.mierunski@nordicsemi.no>