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.

Signed-off-by: Alessandro Carminati <acarmina@redhat.com>
Link: https://patch.msgid.link/20250626083809.314842-1-acarmina@redhat.com
Signed-off-by: Mark Brown <broonie@kernel.org>

Author: alessandrocarminati

drivers/regulator/core.c

@@ -5639,6 +5639,7 @@ static void regulator_remove_coupling(struct regulator_dev *rdev)
 				 ERR_PTR(err));
 	}
 
+	rdev->coupling_desc.n_coupled = 0;
 	kfree(rdev->coupling_desc.coupled_rdevs);
 	rdev->coupling_desc.coupled_rdevs = NULL;
 }