regulator: core: Prevent integer underflow

In the Linux kernel, the following vulnerability has been resolved: regulator: core: Prevent integer underflow By using a ratio of delay to poll_enabled_time that is not integer time_remaining underflows and does not exit the loop as expected. As delay could be derived from DT and poll_enabled_time is defined in the driver this can easily happen. Use a signed iterator to make sure that the loop exits once the remaining time is negative.