iio: imu: st_lsm6dsx: substitute ifdef CONFIG_PM with __maybe_unused macro

Get rid of #ifdef CONFIG_PM by adding __maybe_unused macro to
st_lsm6dsx_suspend and st_lsm6dsx_resume function declarations

Signed-off-by: Lorenzo Bianconi <lorenzo.bianconi@st.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>