kvm: x86: hyperv: delete dead code in kvm_hv_hypercall()

"rep_done" is always zero so the "(((u64)rep_done & 0xfff) << 32)"
expression is just zero.  We can remove the "res" temporary variable as
well and just use "ret" directly.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>