greybus: use a simple list of hd connections

First of all, there's a bug in _gb_hd_connection_insert, which
Viresh found.  But pointing out that problem just called attention
to the fact that I have planning to to remove the affected block of
code.

The set of connections associated with a host device is currently
maintained in a red-black tree.  The number of connections we're
likely to have is on the order of a hundred, and at least for now
isn't even going to approach that.  When this code first went in,
Greg asserted that using a list is speedier than a red-black tree
for smallish numbers of elements (maybe up to a few hundred?).

So this patch just removes the host device's red-black tree of
connections, using a simple list instead.

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