greybus: connection: fail to bind if connection init fails

gb_connection_init() can fail and will return proper error code in that
case, but the caller is ignoring it currently.

Fix that by properly handling errors returned from gb_connection_init()
and propagating them to callers of gb_connection_bind_protocol().

Signed-off-by: Fabien Parent <fparent@baylibre.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>