iio: hid-sensors: avoid unused function warning

A small rework of the PM code in this driver introduced a harmless
warning when CONFIG_PM_SLEEP is not set:

drivers/iio/common/hid-sensors/hid-sensor-trigger.c:212:12: error: 'hid_sensor_resume' defined but not used [-Werror=unused-function]

This removes the #ifdef and instead marks all three PM functions
as __maybe_unused, which covers all possible cases and is harder
to get wrong.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: 7f6cf741 ("iio: hid-sensors: use asynchronous resume")
Signed-off-by: Jonathan Cameron <jic23@kernel.org>