RISC-V: Add a non-void return for sbi v02 functions

SBI v0.2 functions can return an error code from SBI implementation.
We are already processing the SBI error code and coverts it to the Linux
error code.

Propagate to the error code to the caller as well. As of now, kvm is the
only user of these error codes.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>