Apparently, this value is reserved and may be interpreted as changing
doorbell ownership. Even though we're not observing any side effects
now, let's skip over it to be consistent with the spec.

v2: Apply checkpatch (Sagar)
Suggested-by: NSagar Arun Kamble <sagar.a.kamble@intel.com>
Signed-off-by: NMichał Winiarski <michal.winiarski@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Reviewed-by: NSagar Arun Kamble <sagar.a.kamble@intel.com>
Signed-off-by: NChris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20171025200020.16636-2-michal.winiarski@intel.com