regulator: core: fix NULL dereference on unbind due to stale coupling data

In the Linux kernel, the following vulnerability has been resolved: regulator: core: fix NULL dereference on unbind due to stale coupling data Failing to reset coupling_desc.n_coupled after freeing coupled_rdevs can lead to NULL pointer dereference when regulators are accessed post-unbind. This can happen during runtime PM or other regulator operations that rely on coupling metadata. For example, on ridesx4, unbinding the 'reg-dummy' platform device triggers a panic in regulator_lock_recursive() due to stale coupling state. Ensure n_coupled is set to 0 to prevent access to invalid pointers.