xhci: Use device_iommu_mapped()

Replace the dev->iommu_group check with a proper function
call that better reprensents its purpose.

Cc: Mathias Nyman <mathias.nyman@intel.com>
Acked-by: Robin Murphy <robin.murphy@arm.com>
Acked-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>