mlxbf_gige: Enable the GigE port in mlxbf_gige_open

At the moment, the GigE port is enabled in the mlxbf_gige_probe
function. If the mlxbf_gige_open is not executed, this could cause
pause frames to increase in the case where there is high backgroud
traffic. This results in clogging the port.
So move enabling the OOB port to mlxbf_gige_open.

Fixes: f92e1869

 ("Add Mellanox BlueField Gigabit Ethernet driver")
Reviewed-by: David Thompson <davthompson@nvidia.com>
Signed-off-by: Asmaa Mnebhi <asmaa@nvidia.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>