dsp: Check ec ref rx port against AFE_PORT_INVALID

ec ref rx port is defined as integer and it is
set to 0xFFFF (AFE_PORT_INVALID) when ec controls are reset.
Due to this the current check against -1 (0xFFFFFFFF) in
adm open will always return true even after controls are reset.
This leads to adm open failures in case no ec ref channels are
set as part of record usecase. Avoid this by checking ec ref rx
port against AFE_PORT_INVALID.

Change-Id: I0d52bbe2a980d495933239315e6bba193a18567e
Signed-off-by: Chaithanya Krishna Bacharaju <chaithan@codeaurora.org>