aoc/alsa: fix implicit fall-through error

Fixes the following -Wimplicit-fallthrough error:

  aoc/alsa/aoc_alsa_hw.c:116:2:
  error: unannotated fall-through between switch labels [-Werror,-Wimplicit-fallthrough]
        case PORT_INTERNAL_MIC:
        ^
  aoc/alsa/aoc_alsa_hw.c:116:2:
  note: insert '__attribute__((fallthrough));' to silence this warning
        case PORT_INTERNAL_MIC:
        ^
        __attribute__((fallthrough));
  aoc/alsa/aoc_alsa_hw.c:116:2:
  note: insert 'break;' to avoid fall-through
        case PORT_INTERNAL_MIC:
        ^
        break;

Bug: 215617713
Signed-off-by: Carlos Llamas <cmllamas@google.com>
Change-Id: I195dca6f943714fe29683acafa1736c4a611e8ca