target/riscv: fix SBI getchar handler for KVM

target/riscv: fix SBI getchar handler for KVM

Character must be returned via ret[0] field (copied to a0 by KVM).

Return value should be set to 0 to indicate successful processing.

Signed-off-by: Vladimir Isaev <vladimir.isaev@syntacore.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-Id: <20230203135155.12449-1-vladimir.isaev@syntacore.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>