mlxsw: Update specification of reg_ritr_type

The comments really belong to the individual enumerators. The comment
at the register should instead reference the enum.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>