greybus: gpbridge: make gpbridge device ids start with 1

Making gpb device ids consistent with all other devices on
greybus

Testing Done:
Tested using gpb module to make sure the first gpbX/ device
starts with 1.

Signed-off-by: Sandeep Patil <sspatil@google.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Vaibhav Hiremath <vaibhav.hiremath@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>