regulator: tps51632: Get regulator name from i2c_client

Commit "i2c: core: make it possible to match a pure device tree driver"
changed semantics of the i2c probing for device tree devices.
Device tree probed devices now get a NULL i2c_device_id pointer.
This causes the regulator name to be set to NULL and the regulator
registration to fail.

Signed-off-by: Mikko Perttunen <mperttunen@nvidia.com>
Signed-off-by: Mark Brown <broonie@linaro.org>