comment and code discrepancy

In FPU.scala:
```
  // we don't currently support round-max-magnitude (rm=4)
  io.illegal_rm := io.inst(14,12).isOneOf(5.U, 6.U) || io.inst(14,12) === 7.U && io.fcsr_rm >= 5.U

```
It seems that rm=4(round to nearesr, with tie to Max magnitude) is currently supported.
