hal: fix variable initialization

- in case of failure before frame_size declaration, code will jump
  to exit label which will result in error.
- fix this by declaring variable at the beginning of the function

Bug: 37197465
Test: Non-offload PCM playback on Marlin
Change-Id: I9cf83bd34d9f777b605b8d0d44a46a0ab6be802b
Signed-off-by: David Lin <dtwlin@google.com>