ibmveth: Fix oops on request_irq failure

If request_irq fails, the ibmveth driver will overwrite
the rc and end up returning a successful rc on its open
function, resulting in an oops later when a packet gets
sent and buffers are not allocated due to the failed open.

Signed-off-by: Brian King <brking@linux.vnet.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>