device.h: Fix struct member documentation

Fix warning of make xmldocs of documention of the struct member iommu_ops from struct bus_type.

Signed-off-by: Marcos Paulo de Souza <marcos.mage@gmail.com>
Acked-by: Randy Dunlap <rdunlap@xenotime.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>