staging: iio: tsl2x7x_core: Fix sparse warning

Silences the following warning:
drivers/staging/iio/light/tsl2x7x_core.c:553:70:
warning: Using plain integer as NULL pointer

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Cc: J. August Brenner <jbrenner@taosinc.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>