ena: Clear NS_MOREFRAG flag for last netmap slot

When processing packets within the rx-flow
ena_netmap_rx_load_desc doesn't know the number of descriptors, so it
sets NS_MOREFRAG to all the slots to indicate that there are more
fragments for this packet.
The code calls ena_netmap_rx_load_desc() for every descriptor in
this packet to map the relevant buffer into the netmap shared memory.
After ena_netmap_rx_load_desc() calls, we need to unset the NS_MOREFRAG
for the last fragment to indicate that this is the last fragment,
so we explicitly turn off NS_MOREFRAG flag.
Current code overrides all other flags and sets NS_BUF_CHANGED.
This patch unsets the relevant flag only.

Approved by: cperciva (mentor)
MFC after: 2 weeks
Sponsored by: Amazon, Inc.