mlxsw: spectrum_trap: Move policer initialization to mlxsw_sp_trap_init()

No need to initialize a single policer multiple times for each group.
So move the initialization to be done from mlxsw_sp_trap_init(), making
the function much simpler. Also, rename it so it is with sync with
spectrum.c policers initialization.

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