regulator: max8973: Remove unused field from 'struct max8973_chip'

The 'struct regulator_dev *rdev' of 'struct max8973_chip' isn't used
anywhere in the driver so it can be removed safely.

Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Mark Brown <broonie@linaro.org>