bridge: Add bridge API to access the bridge slave port

This API is needed by the NSS connection manager modules
to enable NSS fast path for bridge flows. The connection
manager needs to inspect the netdevice of the bridge slave
port that has learned a given MAC address.
Currently no such API exists that is exported out of the
kernel network stack for this purpose.

Change-Id: I3f8848739d820346135b09b93f760122874468fd
Signed-off-by: Ravinder konka <rkonka@codeaurora.org>
Signed-off-by: Mohammed Javid <mjavid@codeaurora.org>