target/riscv/csr.c: fix 'ret' deadcode in rmw_xireg()

target/riscv/csr.c: fix 'ret' deadcode in rmw_xireg()

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

 > 2706     done:
 > 2707         if (ret) {
 > 2708             return (env->virt_enabled && virt) ?
 > 2709                    RISCV_EXCP_VIRT_INSTRUCTION_FAULT : RISCV_EXCP_ILLEGAL_INST;
 > 2710         }
 >>>>      CID 1590356:  Control flow issues  (DEADCODE)
 >>>>      Execution cannot reach this statement: "return RISCV_EXCP_NONE;".
 > 2711         return RISCV_EXCP_NONE;

Our label is now reduced after fixing another deadcode in the previous
patch but the problem reported here still remains:

 done:
    if (ret) {
        return RISCV_EXCP_ILLEGAL_INST;
    }
    return RISCV_EXCP_NONE;

This happens because 'ret' changes only once at the start of the
function:

    ret = smstateen_acc_ok(env, 0, SMSTATEEN0_SVSLCT);
    if (ret != RISCV_EXCP_NONE) {
        return ret;
    }

So it's a guarantee that ret will be RISCV_EXCP_NONE (-1) if we ever
reach the label, i.e. "if (ret)" will always be true, and  the label can
be even further reduced to:

done:
    return RISCV_EXCP_ILLEGAL_INST;

To make a better use of the label, remove the 'else' from the
xiselect_aia_range() chain and let it fall-through to the 'done' label
since they are now both returning RISCV_EXCP_ILLEGAL_INST.

Resolves: Coverity CID 1590356
Fixes: dc0280723d ("target/riscv: Decouple AIA processing from xiselect and xireg")
Signed-off-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250121184847.2109128-3-dbarboza@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>