dsp: fix configuration of EC reference

The check open_v8.endpoint_id_2 != 0xFFFF can only
be true if changed after initializing open_v8.endpoint_id_2
to 0xFFFF. If not, EC reference will never be configured
correctly. Fixing code to account for that.

Change-Id: I8d2aba19ae1f60ace118c828a98d5fecb149be18
Signed-off-by: Dieter Luecking <dieterl@codeaurora.org>