net/mlx5: Get RoCE netdev

Used by IB driver for determining the IB bond
device's netdev, when LAG is active.

Returns PF0's netdev if mode is not active-backup,
or the PF netdev of the active slave when mode is
active-backup.

Signed-off-by: Aviv Heller <avivh@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>