asoc: fix silence data during concurrent capture

Remove static definition of size and index variables, which are
unique to each capture session. Static defintion of index variable
may incorrectly copy data from wrong memory offset. This might inject
silence/clutter in data copied to audio HAL.

CRs-Fixed: 2457887
Change-Id: I7b2f48d21545655ddaa13566ab4ea87a1cbd7424
Signed-off-by: Vignesh Kulothungan <vigneshk@codeaurora.org>