net/mlx4_core: Fix backward compatibility on VFs

Commit 85743f1e ("net/mlx4_core: Set UAR page size to 4KB regardless
of system page size") introduced dependency where old VF drivers without
this fix fail to load if the PF driver runs with this commit.

To resolve this add a module parameter which disables that functionality
by default.  If both the PF and VFs are running with a driver with that
commit the administrator may set the module param to true.

The module parameter is called enable_4k_uar.

Fixes: 85743f1e

 ('net/mlx4_core: Set UAR page size to 4KB ...')
Signed-off-by: Eli Cohen <eli@mellanox.com>
Tested-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>