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 <[email protected]>