net: mana: Change the function signature of mana_get_primary_netdev_rcu

net: mana: Change the function signature of mana_get_primary_netdev_rcu

Change mana_get_primary_netdev_rcu() to mana_get_primary_netdev(), and
return the ndev with refcount held. The caller is responsible for dropping
the refcount.

Also drop the check for IFF_SLAVE as it is not necessary if the upper
device is present.

Signed-off-by: Long Li <longli@microsoft.com>
Link: https://patch.msgid.link/1741821332-9392-1-git-send-email-longli@linuxonhyperv.com
Signed-off-by: Leon Romanovsky <leon@kernel.org>