regulator: fix crash caused by null driver data

dev_set_drvdata() needs to be called before device_register()
exposes device to userspace. Otherwise kernel crashes after it
gets null pointer from dev_get_drvdata() when userspace tries
to access sysfs entries.

[Removed backtrace for length -- broonie]

Signed-off-by: Yu Zhao <yuzhao@google.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org