regulator: fix use after free bug

This is caused by dereferencing 'rdev' after device_unregister() in
the regulator_unregister() function.  'rdev' is freed by
device_unregister(), so it must not be dereferenced after this call.

[Edited commit message for legibility -- broonie]

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>