amd-xgbe: Call netif_napi_del on ndo_stop operation

Currently the napi context is added using netif_napi_add each time
the ndo_open operation is called.  However, there is not a
corresponding netif_napi_del call during the ndo_stop operation. If
the device ndo_open operation was called more than once an infinite
loop occurs during module unload.  Add a call to netif_napi_del during
the ndo_stop operation.

Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>