ALSA: echoaudio: Remove redundant check

This check is always false, as it's not the responsibilty of the
device-specific code to make this check. It is already checked
in snd_echo_digital_mode_put.

I do not have a Mona interface to test this change.

This patch is in preparation for follow-up patch to modify the
behavior of "opencount".

Signed-off-by: Mark Hills <mark@xwax.org>
Link: https://lore.kernel.org/r/20200708101848.3457-1-mark@xwax.org
Signed-off-by: Takashi Iwai <tiwai@suse.de>