IB/ipoib: Remove the unnecessary error check

The function ipoib_mcast_start_thread/ipoib_ib_dev_up always return zero.
As such, in the function ipoib_open, err_stop will never be reached.
So remove this err_stop and change the return type of the function
ipoib_mcast_start_thread/ipoib_ib_dev_up to void.

Signed-off-by: Zhu Yanjun <yanjun.zhu@oracle.com>
Reviewed-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>