mlx4: restore conditional call to napi_complete_done()

After commit 1a288172

 ("mlx4: use napi_complete_done()") we ended up
calling napi_complete_done() in the case NAPI poll consumed all its
budget.

This added extra interrupt pressure, this patch restores proper
behavior.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Fixes: 1a288172

 ("mlx4: use napi_complete_done()")
Signed-off-by: David S. Miller <davem@davemloft.net>