lib.sh: switch MTL back to default MAX_WAIT_FW_LOADING=3 seconds

i915 does not hang MTL any more, this has been fixed a long time ago.

On the other hand, 70s is a very long time to wait when the firmware
fails to load for some reason; this is wasting precious cycles. See
recent example in:
  https://sof-ci.01.org/linuxpr/PR4995/build3008/devicetest/index.html
  #1112 (comment)

Signed-off-by: Marc Herbert <marc.herbert@intel.com>

case-lib/lib.sh

@@ -89,11 +89,14 @@ start_test()
  if test -z "$MAX_WAIT_FW_LOADING"; then
  local _pltf; _pltf=$("$SCRIPT_HOME/tools/sof-dump-status.py" -p)
  case "$_pltf" in
+
  # broken i915 with long timeout, see comments in config.sh
- mtl) MAX_WAIT_FW_LOADING=70;;
+ # add_slow_prototype_platform_here) MAX_WAIT_FW_LOADING=70;;
+
  # rt1011 i2c-10EC1011:00 needs 10 seconds on cml-hel-rt5682,
  # see https://github.com/thesofproject/sof-test/pull/1095
  cml) MAX_WAIT_FW_LOADING=13;;
+
  *) MAX_WAIT_FW_LOADING=3;;
  esac
  fi