regulator: max77686: Check pdata->num_regulators earlier

If this driver only works when pdata->num_regulators == MAX77686_REGULATORS,
let's check it earlier.

Also remove unused num_regulators from struct max77686_data.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>