octeontx2-af: Check return value of nix_get_nixlf before using nixlf

If a NIXLF is not attached to a PF/VF device then
nix_get_nixlf function fails and returns proper error
code. But npc_get_default_entry_action does not check it
and uses garbage value in subsequent calls. Fix this
by cheking the return value of nix_get_nixlf.

Fixes: 967db352 ("octeontx2-af: add support for multicast/promisc packet replication feature")
Signed-off-by: Subbaraya Sundeep <sbhatta@marvell.com>
Signed-off-by: David S. Miller <davem@davemloft.net>