mfd: wm8994: Fix NULL pointer exception on missing pdata

If we use the dev_get_platdata, pdata can be NULL in DT environments.
Actually, NULL pointer exception is occurs with error messages like this:

[    1.226583] Unable to handle kernel NULL pointer dereference at virtual address 00000044
[    1.467910] [<c02f1e38>] (wm8994_irq_init) from [<c02f1bd4>] (wm8994_i2c_probe+0x684/0x858)
[    1.476241] [<c02f1bd4>] (wm8994_i2c_probe) from [<c0371c78>] (i2c_device_probe+0x11c/0x17c)

In wm8994-core, we already checked whether platform data is NULL and set the
wm8994->pdata to platform data if platform data is not NULL.
So, we can use wm8994->pdata instead of platform data.

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>