RISC-V: KVM: Use switch-case in kvm_riscv_vcpu_set/get_reg()

We should use switch-case in kvm_riscv_vcpu_set/get_reg() functions
because the else-if ladder is quite big now.

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