xen/xenbus: Correct the comments for xenbus_grant_ring()
xen/xenbus: Correct the comments for xenbus_grant_ring()

A grant reference (which is a positive number) can indicate success, so
the original comments need be improved.

Signed-off-by: Chen Gang <080e0242401949f8c1dfb399e414be816846daae@gmail.com>
Signed-off-by: David Vrabel <b9d45bd1f671e508cd8daa63dfc3cabb597562df@citrix.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs