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>