net: calxedaxgmac: enable interrupts after napi_enable

Fix a race condition where the interrupt handler may have called
napi_schedule before napi_enable is called. This would disable interrupts
and never actually schedule napi to run.

Reported-by: Lennert Buytenhek <buytenh@wantstofly.org>
Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: David S. Miller <davem@davemloft.net>