mfd: Add missing max8997 static storage class specifier

Fixes the following sparse warnings:
drivers/mfd/max8997.c:209:4: warning: symbol 'max8997_dumpaddr_pmic' was not declared. Should it be static?
drivers/mfd/max8997.c:334:4: warning: symbol 'max8997_dumpaddr_muic' was not declared. Should it be static?
drivers/mfd/max8997.c:344:4: warning: symbol 'max8997_dumpaddr_haptic' was not declared. Should it be static?
drivers/mfd/max8997.c:426:25: warning: symbol 'max8997_pm' was not declared. Should it be static?

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: MyungJoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>