ASoC: core: Don't overwrite .poweroff in snd_soc_pm_ops

SET_SYSTEM_SLEEP_PM_OPS writes .poweroff = *_resume once. Then we overwrite it
again explicitly as .poweroff = snd_soc_poweroff. Even though it works, as the
second one overwrites the first one, this is not the correct way. Fix this by
expanding SET_SYSTEM_SLEEP_PM_OPS in our structure.

Signed-off-by: Viresh Kumar <viresh.kumar@st.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>