xen/iommu: dt: Check the return value of xsm_deassign_dtdevice()

xen/iommu: dt: Check the return value of xsm_deassign_dtdevice()

xsm_deassign_dtdevice() will indicate whether the caller is allowed
to issue the operation. So the return value has to be checked.

Spotted by clang static analyzer.

Fixes: fe36cccc483c ("xen/passthrough: Extend XEN_DOMCTL_*assign_device to support DT device")
Signed-off-by: Julien Grall <jgrall@amazon.com>
Reviewed-by: Michal Orzel <michal.orzel@arm.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>