mfd: wm8994: Set mfd id-base for regulator devs creation to avoid conflicts

After commit: 6e3f62f0

 ("mfd: core: Fix platform-device id generation")
We must set the id base when register a duplicate name of mfd_cell.
If not, duplicate filename error was reported.

- sysfs: cannot create duplicate filename '/devices/.../wm8994-ldo'

Signed-off-by: Inha Song <ideal.song@samsung.com>
Acked-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>