mfd: wm8994-core: Naturalise cross-architecture discrepancies

If we compile the WM8994 for a 64bit architecture we receive the following
warnings:

drivers/mfd/wm8994-core.c: In function ‘wm8994_i2c_probe’:
drivers/mfd/wm8994-core.c:639:19:
  warning: cast from pointer to integer of different size
    wm8994->type = (int)of_id->data;
                   ^

Signed-off-by: Lee Jones <lee.jones@linaro.org>