ALSA: platform: Check CONFIG_PM_SLEEP instead of CONFIG_PM

When CONFIG_PM is set but CONFIG_PM_SLEEP is unset,
SIMPLE_DEV_PM_OPS() ignores the given functions, and this leads to
compile warnings.

For avoiding this, simply check CONFIG_PM_SLEEP instead of CONFIG_PM.

Reported-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>