target/riscv/csr.c: fix deadcode in rmw_xiregi()

target/riscv/csr.c: fix deadcode in rmw_xiregi()

Coverity found a DEADCODE issue in rmw_xiregi() claiming that we can't
reach 'RISCV_EXCP_VIRT_INSTRUCTION_FAULT' at the 'done' label:

 > 2652     done:
 >>>>      CID 1590357:  Control flow issues  (DEADCODE)
 >>>>      Execution cannot reach the expression "RISCV_EXCP_VIRT_INSTRUCTION_FAULT"
     inside this statement: "return (env->virt_enabled &...".
 > 2653         return (env->virt_enabled && virt) ?
 > 2654                 RISCV_EXCP_VIRT_INSTRUCTION_FAULT : RISCV_EXCP_ILLEGAL_INST;

This happens because 'virt' is being set to 'false' and it will remain
as 'false' in any code path where 'done' will be called. The label can
be safely reduced to:

done:
     return RISCV_EXCP_ILLEGAL_INST;

And that will leave us with the following usage of a 'goto' skipping a
single 'return' to do another single 'return':

     } else {
        goto done;
     }

     return rmw_xireg_csrind(env, csrno, isel, val, new_val, wr_mask);

done:
     return RISCV_EXCP_ILLEGAL_INST;

Which we will eliminate it and just do 'return RISCV_EXCP_ILLEGAL_INST'
instead.

Resolves: Coverity CID 1590357
Fixes: 5e33a20827 ("target/riscv: Support generic CSR indirect access")
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250121184847.2109128-4-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>