regulator: Fix rc5t583_regulator_probe error handling

1. regulator_register returns ERR_PTR on error, thus use IS_ERR to check the
   return value.
2. Fix off-by-one for unregistering the registered regulator.
   Current code does not unregister regs[0].rdev in clean_exit.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>