regulator: core: return err value for regulator_get if there is no DT binding

commit 6d191a5f
(regulator: core: Don't defer probe if there's no DT binding for a supply)

Attempted to differentiate between regulator_get() with an actual
DT binding for the supply and when there is none to avoid unnecessary
deferal.
However, ret value supplied by regulator_dev_lookup() is being
ignored by regulator_get(). So, exit with the appropriate return value.

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>