Call rtnl_unlock() on this error path, before returning.

Fixes: bc23aa94 ("igc: Add pcie error handler support")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Gerhard Engleder <gerhard@engleder-embedded.com>
Signed-off-by: David S. Miller <davem@davemloft.net>