alsa_sound: Fix for Tunnelplayer IOCTL drain block

-IOCTL drain was getting stuck when standby was
called after seeking to EOS
-This happens when seek was done to the
end of the stream which would lead to only
a zero length buffer being sent.
In this case, IOCTL drain will block forever
-To solve this, IOCTL drain was interrupted
by calling flush from the context of standby

CRs-Fixed: 467166

Change-Id: I0d119c362670fd4f99b023dd4a1fdebde7832c02