iio: tsl4531: Fix compiler error when CONFIG_PM_OPS is not defined

Fix the compiler error when the CONFIG_PM_OPS flag is not set.

drivers/iio/light/tsl4531.c:235:8: error: ‘tsl4531_suspend’ undeclared here (not in a function)
drivers/iio/light/tsl4531.c:235:8: error: ‘tsl4531_resume’ undeclared here (not in a function)

Signed-off-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>