bridge: add missing bridge port check for offloads

This patch fixes a missing bridge port check caught by smatch.

setlink/dellink of attributes like vlans can come for a bridge device
and there is no need to offload those today. So, this patch adds a bridge
port check. (In these cases however, the BRIDGE_SELF flags will always be set
and we may not hit a problem with the current code).

smatch complaint:

The patch 68e331c7: "bridge: offload bridge port attributes to
switch asic if feature flag set" from Jan 29, 2015, leads to the
following Smatch complaint:

net/bridge/br_netlink.c:552 br_setlink()
	 error: we previously assumed 'p' could be null (see line 518)

net/bridge/br_netlink.c
   517
   518		if (p && protinfo) {
                    ^
Check for NULL.

Reported-By: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>