mlxsw: spectrum_router: Handle sysctl_ip_fwd_update_priority

This sysctl setting controls whether packet priority should be updated
after forwarding. Configure RGCR.usp accordingly so that the device is
in sync with the kernel handling.

Note that RGCR doesn't allow changing arbitrary parameters
mid-operation, however "usp" is exempt and can be reconfigured.

Also react to NETEVENT_IPV4_FWD_UPDATE_PRIORITY_UPDATE notifications
that signify change in this configuration.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>