iommu/amd: Don't clear DTE flags when modifying it

During device assignment/deassignment the flags in the DTE
get lost, which might cause spurious faults, for example
when the device tries to access the system management range.
Fix this by not clearing the flags with the rest of the DTE.

Reported-by: G. Richard Bellamy <rbellamy@pteradigm.com>
Tested-by: G. Richard Bellamy <rbellamy@pteradigm.com>
Cc: stable@vger.kernel.org
Signed-off-by: Joerg Roedel <jroedel@suse.de>