regulator: max8998 - fix memory allocation size for max8998->rdev

We only use max8998->rdev[0] .. max8998->rdev[pdata->num_regulators-1],
max8998->rdev[pdata->num_regulators] is not used.
Thus fix the memory allocation size.

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>