xen/evtchn_fifo: fix error return code in evtchn_fifo_setup()

Fix to return -ENOMEM from the error handling case instead of
0 (overwrited to 0 by the HYPERVISOR_event_channel_op call),
otherwise the error condition cann't be reflected from the
return value.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>