RDMA/mana_ib: Introduce mana_ib_get_netdev helper function

Use a helper function to access netdevs using a port number.
This patch removes code repetitions as well as removes the need
to explicitly use gdma_dev, which was error-prone.

Signed-off-by: Konstantin Taranov <kotaranov@microsoft.com>
Link: https://lore.kernel.org/r/1705965781-3235-3-git-send-email-kotaranov@linux.microsoft.com


Signed-off-by: Leon Romanovsky <leon@kernel.org>