net/enic: fix assignment

As it stands, the existing assignment to mbuf has no effect outside of
the function.  Prior to this change, the mbuf argument would contain
an invalid address, but it would not be null.  After this change, the
caller gets a null mbuf back.

Fixes: 947d860c821f ("enic: improve Rx performance")
Cc: stable@dpdk.org
Signed-off-by: Aaron Conole <aconole@redhat.com>
Reviewed-by: John Daley <johndale@cisco.com>