[prim_arbiter_ppc] Fix lint error
[prim_arbiter_ppc] Fix lint error

Fix lint error reported by AscentLint:

```
E   SIGNED_RANGE:   prim_arbiter_ppc.sv:123   Part select of signed signal 'i[IdxW - 1:0]' encountered                 New
```

Signed-off-by: Philipp Wagner <5a4b10454c30419e54a1570be358a516ff84bb63@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan