regulator: max8952 - fix wrong gpio valid check

Signed-off-by: Jonghwan Choi <jhbird.choi@samsung.com>
Acked-by: MyungJoo Ham <myungjoo.ham@samsung.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>