iio: common: ssp_sensors: Protect PM-only functions to kill warning

If CONFIG_PM_SLEEP=n:

    drivers/iio/common/ssp_sensors/ssp_dev.c:644: warning: ‘ssp_suspend’ defined but not used
    drivers/iio/common/ssp_sensors/ssp_dev.c:669: warning: ‘ssp_resume’ defined but not used

Protect the unused functions by #ifdef CONFIG_PM_SLEEP to fix this.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Acked-by: Karol Wrona <k.wrona@samsung.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>