greybus: bootrom: Create gb_bootrom_cancel_timeout()

We set timeouts using gb_bootrom_set_timeout(), which hides the internal
implementation, i.e. workqueues.

While canceling timeouts, we do cancel_delayed_work_sync(), which
exposes the internal implementation and doesn't look that clean.

Create gb_bootrom_cancel_timeout() to hide the internal implementation
here as well.

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>