serial: st-asc: 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>
Link: https://lore.kernel.org/r/20240716180010.126987-1-rgallaispou@gmail.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>