greybus: interface: fix double freeing of interface structure

The interface will be released by gb_interface_release() once all
references for the interface are dropped. And so there is no need to
free it in the error path specially.

Reviewed-by: Johan Hovold <johan@hovoldconsulting.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>