RISC-V: KVM: Return correct code for hsm stop function

According to the SBI specification, the stop function can only
return error code SBI_ERR_FAILED. However, currently it returns
-EINVAL which will be mapped SBI_ERR_INVALID_PARAM.

Return an linux error code that maps to SBI_ERR_FAILED i.e doesn't map
to any other SBI error code. While EACCES is not the best error code
to describe the situation, it is close enough and will be replaced
with SBI error codes directly anyways.

Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Atish Patra <atishp@rivosinc.com>
Signed-off-by: Anup Patel <anup@brainfault.org>