forcedeth: Fix invalid errno reporting in nv_request_irq()

Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Patrick McHardy <kaber@trash.net>
Cc: netdev@vger.kernel.org
Cc: linux-pci@vger.kernel.org
Signed-off-by: David S. Miller <davem@davemloft.net>