regulator: ad5398 - fix a memory leak

In current implementation, the address return from regulator_register()
is different from the address for regulator_unregister().

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Sonic Zhang <sonic.zhang@analog.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>