mlxsw: reg: Rename the flexible action set length field

The MLXSW_REG_PXXX_FLEX_ACTION_SET_LEN is relevant for the multicast router
registers too, so rename it to have a general name which is not bound to a
specific register.

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