RISC-V: KVM: Make kvm_riscv_guest_timer_init a void function

It can never fail so convey that fact explicitly by making the function
void. Also in kvm_arch_init_vm it makes it clear that there no need
to do any cleanup after kvm_riscv_gstage_vmid_init has been called.

Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Anup Patel <anup@brainfault.org>