ntb: intel: Make intel_ntb3_peer_db_addr static

Fix sparse warning:

drivers/ntb/hw/intel/ntb_hw_gen3.c:535:5: warning:
 symbol 'intel_ntb3_peer_db_addr' was not declared. Should it be static?
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Acked-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>

drivers/ntb/hw/intel/ntb_hw_gen3.c

@@ -532,7 +532,7 @@ static int intel_ntb3_mw_set_trans(struct ntb_dev *ntb, int pidx, int idx,
 	return 0;
 }
 
-int intel_ntb3_peer_db_addr(struct ntb_dev *ntb, phys_addr_t *db_addr,
+static int intel_ntb3_peer_db_addr(struct ntb_dev *ntb, phys_addr_t *db_addr,
 				   resource_size_t *db_size,
 				   u64 *db_data, int db_bit)
 {