net/mlx5e: Fix possible race condition in macsec extended packet number update routine

Currenty extended packet number (EPN) update routine is accessing
macsec object without holding the general macsec lock hence facing
a possible race condition when an EPN update occurs while updating
or deleting the SA.
Fix by holding the general macsec lock before accessing the object.

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