fix(prototyper): use `already_available` when target hart is already started in HSM extension (#52)

In the RISC-V SBI Specification, it shows that `SBI_ERR_ALREADY_AVAILABLE` represents 'the given hartid is already started'. The `SBI_ERR_ALREADY_STARTED` error code is used when a PMU counter or a SSE software event is started.

Ref: https://github.com/riscv-non-isa/riscv-sbi-doc/blob/master/src/ext-hsm.adoc#function-hart-start-fid-0

Signed-off-by: Zhouqi Jiang <luojia@hust.edu.cn>

prototyper/src/sbi/hsm.rs

@@ -200,7 +200,7 @@ impl rustsbi::Hsm for SbiHsm {
                     }
                     SbiRet::success(0)
                 } else {
-                    SbiRet::already_started()
+                    SbiRet::already_available()
                 }
             }
             None => SbiRet::invalid_param(),