ASOC : Remove .flush function of voice SVC driver

IMS_RTP_D uses Popen and Pclose for obtaining IP address.
Popen creates a child process by fork(). This creates
a copy of audio FD. This new audio FD is closed on
child process termination. Hence flush is invoked.
Move .flush() implementation to .release() to avoid closing
CVS and MVM handles inside audio driver when child process
terminates.

CRs-fixed: 686488
Change-Id: I7e2cbd5aab986be7f9698a2ed564fafa404009fc
Signed-off-by: Karthik Reddy Katta <a_katta@codeaurora.org>