greybus: match AP CPort ids with module

We have no control protocol set up for communicating CPort ids
between connections.  Specifically, the module firmware has no way
of knowing what CPort id the AP is using for any connection between
the two.  Alexandre decided to just assume the CPort id on the AP
side would be the same as the one on the module side, and this
assumption works pretty well for our simple demo.

This patch defines gb_connection_hd_cport_id_alloc_one(), which is
used to allocate a particular AP side CPort id to assign for use for
a new connection.  This is used in gb_connection_create() to
request the CPort id supplied in a module manifest.

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