net/mlx5: Cache the system image guid

The system image guid is a read-only field which is used by the TC
offloads code to determine if two mlx5 devices belong to the same
ASIC while adding flows.

Read this once and save it on the core device rather than querying each
time an offloaded flow is added.

Signed-off-by: Alaa Hleihel <alaa@mellanox.com>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>