regulator: max77693: Use core code for charger's is_enabled

The custom implementation of 'regulator_ops.is_enabled' callback for
charger regulator is exactly the same as regulator_is_enabled_regmap()
with 'enable_val' set.

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