mlx5: Enable mutual support for IB and Ethernet

Ethernet functionality is only available when working in ISSI > 0 mode.

Previously, the IB driver wasn't ready to work on that mode, and hence
building both the IB driver and the Ethernet functionality in the core
driver were disallowed by Kconfigs.

Now, once we have all the pre-steps in place, we can remove this limitation.

The last steps in the IB driver for getting that setup to work are:
create dummy SRQ for the driver's use (until now we could use XRC_SRQ
as SRQ and XRC_SRQ, after moving to ISSI > 0, we separate XRC SRQs from
basic SRQs) and adapt the create QP function to be compatible with ISSI > 0.

Signed-off-by: Haggai Abramovsky <hagaya@mellanox.com>
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>