net: macb: Only disable NAPI on the actual error path

A recent change added a disable to NAPI into macb_open, this was
intended to only happen on the error path but accidentally applies
to all paths. This causes NAPI to be disabled on the success path, which
leads to the network to no longer functioning.

Fixes: 014406ba

 ("net: cadence: macb: disable NAPI on error")
Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Tested-by: Corentin Labbe <clabbe@baylibre.com>
Signed-off-by: David S. Miller <davem@davemloft.net>