mfd: Staticise non-exported symbols in MAX8998 driver

No need to have them in the global namespace and sparse complains.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>