net/mlx5: E-Switch, Enable chains only if regs loopback is enabled

Register c0 loopback is needed to fully support chains and prios.

Enable chains and prio only if loopback (of reg c1 which came together
with c0), is enabled. To be able to check that, move enabling of loopback
before eswitch chains init.

Signed-off-by: Paul Blakey <paulb@mellanox.com>
Reviewed-by: Roi Dayan <roid@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>