mtd: st_spi_fsm: Switch from CONFIG_PM_SLEEP guards to pm_sleep_ptr()

Letting the compiler remove these functions when the kernel is built
without CONFIG_PM_SLEEP support is simpler and less error prone than the
use of #ifdef based kernel configuration guards.

Signed-off-by: Raphael Gallais-Pou <rgallaispou@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>