xen-platform: fix error return code in platform_pci_init()

Fix to return a negative error code from the error handling
case instead of 0, 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>