thunderbolt: Do not allocate switch if depth is greater than 6

Maximum depth in Thunderbolt topology is 6 so make sure it is not
possible to allocate switches that exceed the depth limit.

While at it update tb_switch_alloc() to use upper/lower_32_bits()
following tb_switch_alloc_safe_mode().

Signed-off-by: Mika Westerberg <mika.westerberg@linux.intel.com>
diff --git a/drivers/thunderbolt/tb.h b/drivers/thunderbolt/tb.h
index f7b0c43..93c1ea2 100644
--- a/drivers/thunderbolt/tb.h
+++ b/drivers/thunderbolt/tb.h
@@ -43,6 +43,7 @@ struct tb_switch_nvm {
 };
 
 #define TB_SWITCH_KEY_SIZE		32
+#define TB_SWITCH_MAX_DEPTH		6
 
 /**
  * struct tb_switch - a thunderbolt switch