net/mlx5e: Fix macsec ssci attribute handling in offload path

Currently when macsec offload is set with extended packet number (epn)
enabled, the driver wrongly deduce the short secure channel identifier
(ssci) from the salt instead of the stand alone ssci attribute as it
should, consequently creating a mismatch between the kernel and driver's
ssci values.
Fix by using the ssci value from the relevant attribute.

Fixes: 4411a6c0 ("net/mlx5e: Support MACsec offload extended packet number (EPN)")
Signed-off-by: Emeel Hakim <ehakim@nvidia.com>
Reviewed-by: Raed Salem <raeds@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>