Use IFLA_AF_SPEC nested attributes to lookup bridge mode and when
doing strcmp() check for equality.

These appear to be typos from the original commit,

commit 64108901
Author: Vlad Yasevich <vyasevic@redhat.com>
Date:   Fri Mar 15 10:01:28 2013 -0700

    bridge: Add support for setting bridge port attributes

Also set flags to BRIDGE_FLAGS_SELF instead of using OR operation.
This allows setting the bridge mode when not being used with a
master device.

To allow setting both master and self devices simultaneously we
will need to add a {self|master} field similar to fdb commands.
For now the command sets are mutually exclusive as noted in the
original commit.

With this patch 'bridge link set' works now,

# ./bridge/bridge link set dev veth1 cost 3
# ./bridge/bridge link show
10: veth1 state UP : <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 master bridge0 state forwarding priority 3 cost 3

CC: Vlad Yasevich <vyasevic@redhat.com>
Signed-off-by: John Fastabend <john.r.fastabend@intel.com>