IB/ipoib: Set device connection mode only when needed

When changing the connection mode, the ipoib_set_mode function
did not check if the previous connection mode equals to the
new one. This commit adds the required check and return 0 if the new
mode equals to the previous one.

Fixes: 839fcaba

 ("IPoIB: Connected mode experimental support")
Signed-off-by: Feras Daoud <ferasda@mellanox.com>
Signed-off-by: Erez Shitrit <erezsh@mellanox.com>
Reviewed-by: Alex Vesker <valex@mellanox.com>
Reviewed-by: Yuval Shaia <yuval.shaia@oracle.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>
Signed-off-by: Doug Ledford <dledford@redhat.com>