greybus: connection: add functions to get/set private data

Add gb_connection_get_data() and gb_connection_set_data() to get and set
the private data of a connection, instead of "open coding" it
everywhere.

Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>