compat: add br_port_exists() and ask for inclusion upstream

compat: add br_port_exists() and ask for inclusion upstream

The br_port_exists() call is used internally on private bridge
code on net/bridge/ to check if a netdevice is a bridge port.
The way to check if a netdevice is a bridge port has changed
for 2.6.36 from checking the netdev->br_port pointer to
checking for a IFF_BRIDGE_PORT flag on the netdev->priv_flags.

Instead of requiring backport code to ifdef the checks lets
move the private br_port_exists() to a public header accessible
by drivers and consistantly use it to check for the bridge
port.

I'll submit a patch upstream to move br_port_exists() to a public
header so that we don't have to later even require a patch on
backported code. We'll see if it gets accepted though.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>