greybus: add bg_hd_connection_find()

greybus: add bg_hd_connection_find()

Add a function that looks up a connection given the host device
pointer an the host cport id.  This will be used to determine which
connection an incoming message is associated with.

Replace the list tracking host device connections with a red-black
tree so lookup can scale and be done quickly.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>