arch/tile: fix bug in delay_backoff()

We were carefully computing a value to use for the number of loops
to spin for, and then ignoring it.
Signed-off-by: NChris Metcalf <cmetcalf@tilera.com>

arch/tile/lib/spinlock_common.h

@@ -60,5 +60,5 @@ static void delay_backoff(int iterations)
 	loops += __insn_crc32_32(stack_pointer, get_cycles_low()) &
 		(loops - 1);
 
-	relax(1 << exponent);
+	relax(loops);
 }