include: dsp: avoid returning error code from stubbed function

If MSM_QDSP6_NOTIFIER is not enabled for a target, audio_notifier
feature will not be available for that target. Whenever a call is
made to audio_notifier when its not supported, returning error from
stubbed function may confuse the error with real failure (even though
the feature is not enabled on the target). Avoid returning error code
from stubbed function, when feature is disabled.

Change-Id: I37eb2e25e45b25948aff55f2a231195a22b16f05
Signed-off-by: Banajit Goswami <bgoswami@codeaurora.org>