gru: check for correct GRU chiplet assignment

Simplify the code that checks for correct assignment of GRU contexts to
users.

Signed-off-by: Jack Steiner <steiner@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>