mfd: max8998: Naturalise cross-architecture discrepancies

If we compile the MAX8998 for a 64bit architecture we receive the following
warnings:

  drivers/mfd/max8998.c: In function ‘max8998_i2c_get_driver_data’:
  drivers/mfd/max8998.c:178:10:
    warning: cast from pointer to integer of different size
     return (int)match->data;
            ^

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