NX90: Writing non-volatile attributes in rapid succession may crash the system

When writing non-volatile attribute values in rapid succession, so that the system fails to keep up to actually perform the flash writes and thus gets into a "no more resources" state, then an unitialized function pointer may be called which will crash the firmware.

It should be investigated how a burst-write of non-volatile data from the network or host side is handled by the system. We should block along the way until these NV-writes are finished. We should not lose write operations of attribute values due to running out of async requests/buffers.