staging: iio: tsl2583: fix unused function warning

Removing a call to the taos_chip_off() makes it unused when CONFIG_PM
is disabled:

drivers/staging/iio/light/tsl2583.c:438:12: error: ‘taos_chip_off’ defined but not used [-Werror=unused-function]

This removes all the #ifdef in this file, and marks the PM functions as
__maybe_unused instead, which is more reliable and gives us better
compile time coverage.

Fixes: 0561155f ("staging: iio: tsl2583: don't shutdown chip when updating the lux table")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>