RISC-V: KVM: Fix error code returned for Smstateen ONE_REG

RISC-V: KVM: Fix error code returned for Smstateen ONE_REG

Return -ENOENT for Smstateen ONE_REG when:
1) Smstateen is not enabled for a VCPU
2) ONE_REG id is out of range

This will make Smstateen ONE_REG error codes consistent
with other ONE_REG interfaces of KVM RISC-V.

Fixes: c04913f2b54e ("RISCV: KVM: Add sstateen0 to ONE_REG")
Signed-off-by: Anup Patel <anup.patel@oss.qualcomm.com>
Link: https://lore.kernel.org/r/20260120080013.2153519-2-anup.patel@oss.qualcomm.com
Signed-off-by: Anup Patel <anup@brainfault.org>