greybus: connection: call gb_connection_exit() from gb_connection_destroy()

Both the routines are always called together and in the same sequence.
Rather than duplicating this at different places, make
gb_connection_destroy() call gb_connection_exit().

This also makes it more sensible, as gb_connection_init() is never
called directly by the users and so its its counterpart shouldn't be
called directly as well.

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