mfd: Fix max8907 sparse warning

This fixes:

drivers/mfd/max8907.c:179:16: sparse: symbol 'max8907_pm_off' was not declared. Should it be static?
Signed-off-by: NFengguang Wu <fengguang.wu@intel.com>
Signed-off-by: NSamuel Ortiz <sameo@linux.intel.com>

drivers/mfd/max8907.c

@@ -176,7 +176,7 @@ static const struct regmap_irq_chip max8907_rtc_irq_chip = {
 	.num_irqs = ARRAY_SIZE(max8907_rtc_irqs),
 };
 
-struct max8907 *max8907_pm_off;
+static struct max8907 *max8907_pm_off;
 static void max8907_power_off(void)
 {
 	regmap_update_bits(max8907_pm_off->regmap_gen, MAX8907_REG_RESET_CNFG,