regulator: max8998 - set max8998->num_regulators

Set max8998->num_regulators = pdata->num_regulators,
otherwise it's default value is 0.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>