regulator: reorder initialization steps in regulator_register()

device_register() is calling ->get_voltage() as part of it's sysfs attribute
initialization process, and this functions might need to know the regulator
constraints to return a valid value.
This is at least true for the pwm regulator driver (when operating in
continuous mode) which needs to know the minimum and maximum voltage values
to calculate the current voltage:

min_uV + (((max_uV - min_uV) * dutycycle) / 100);

Move device_register() after set_machine_constraints() to make sure those
constraints are correctly initialized when ->get_voltage() is called.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Reported-by: Stephen Barber <smbarber@chromium.org>
Tested-by: Caesar Wang <wxt@rock-chips.com>
Signed-off-by: Mark Brown <broonie@kernel.org>