asoc: Fix read offset for compress capture with timestamp

In a compress capture usecase, read offset guides to
move buffer pointer based on buffer length. When timestamp is
enabled read offset also needs to include timestamp header
offset along with buffer length. This fix addresses the issue
of null memory map handle due to incorrect read offset.

Change-Id: I93c3ec588e77d535b6c7a4a0d832c1e7ea5f62a7
Signed-off-by: Vikram Panduranga <vpandura@codeaurora.org>