mlx4_core/ib: set the IB port MTU to 2K

mlx4_core/ib: set the IB port MTU to 2K

'commit 096335b3f983 ("mlx4_core: Allow dynamic MTU configuration for IB
ports")' overwrite the default port MTU and sets it as 4K. Since this
directly impacts the HW VLs supported and Oracle workloads heavily uses
all supported 8 VLs for traffic classification, 2K default needs to
be kept as is.

We initilise it to default 2k so that the feature(dynamic MTU configuration)
is still available for non DB users to set the desired MTU value using sysctl.

Also for CX2 cards, commit  596c5ff4b7b3 ("net/mlx4: adjust initial
value of vl_cap in mlx4_SET_PORT") broke the vl_cap which made the
supported VLs to 4 irrespective of MTU size.

Orabug: 24946479

Tested-by: Pierre Orzechowski <pierre.e.orzechowski@oracle.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@oracle.com>