mlxsw: spectrum: Change default rate and priority of DHCP packets

Reduce the default acceptable rate of DHCP packets to 128 packets per
second and reduce their priority. This is reasonable given the Spectrum
ASICs are limited to 128 ports at the moment.

These are only the default values. Users will be able to modify them via
devlink-trap.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>