iio: magn: ak8975: refactor regulator handlers

Move the regulator_get() calls directly into the probe() function,
keep only the power_on()/power_off() functions to flick the
regulators on/off.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>