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.