xics_kvm: fix a build break

xics_kvm: fix a build break

On CentOS 7.5, gcc-4.8.5-28.el7_5.1.ppc64le fails to build QEMU due to :

  hw/intc/xics_kvm.c: In function ‘ics_set_kvm_state’:
  hw/intc/xics_kvm.c:281:13: error: ‘ret’ may be used uninitialized in this
    function [-Werror=maybe-uninitialized]
             return ret;

Fix the breakage and also remove the extra error reporting as
kvm_device_access() already provides a substantial error message.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>