propagate an error in enabling the PCI device.

Also eliminates this warning:

 arch/x86/kernel/amd_iommu_init.c: In function ‘init_iommu_one’:
 arch/x86/kernel/amd_iommu_init.c:726: warning: ignoring return value of ‘pci_enable_device’, declared with attribute warn_unused_result
Signed-off-by: Ingo Molnar <mingo@elte.hu>