fix for DC Shell

Created by: davideschiavone

IDLE is defined both in the prefetcher FSM and the MULT FSM.

DC Shell does not like that as:

dc_shell> analyze -format sverilog  -work work ${DESIGN_RTL_DIR}/cv32e40p_mult.sv
Running PRESTO HDLC
Compiling source file ../../riscv/rtl//cv32e40p_mult.sv
Error:  ../../riscv/rtl//cv32e40p_mult.sv:96: redeclaration of symbol IDLE as constant. (VER-513)
Error:  ../../riscv/rtl//cv32e40p_mult.sv:96: symbol IDLE must be a constant or parameter. (VER-260)
* Presto compilation terminated with 2 errors. *

This PR solves it