Audio primary hal: fix member function used before init

prepare for write was refactor by Ib4170eb6a9f88f9352d0912083b43d600771bb8e
to use temporary variable in a rai pattern.
Nevertheless the member variable was still used too early in
prepareForRead resulting in a crash for capture.

Test: assistant, camcorder
Bug: 37492059

Change-Id: I08140834959c440798cd40700dd089adca2f3e40
Signed-off-by: Kevin Rocard <krocard@google.com>