greybus: interface: gb_interface_create() isn't called for existing interfaces

The callers are ensuring that another interface doesn't exist with the
same interface id and so there is no need to check that from
gb_interface_create() anymore.

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