mfd: Fix pm8606 build failure

Reported by Andrew Morton <akpm@linux-foundation.org>, with
i386 allmodconfig:

ERROR: "pm8606_osc_disable" [drivers/video/backlight/88pm860x_bl.ko] undefined!
ERROR: "pm8606_osc_enable" [drivers/video/backlight/88pm860x_bl.ko] undefined!
ERROR: "pm8606_osc_disable" [drivers/leds/leds-88pm860x.ko] undefined!
ERROR: "pm8606_osc_enable" [drivers/leds/leds-88pm860x.ko] undefined!

Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>