asoc: refine the probe for loopback devices

msm_pcm_loopback_probe should be conducted only for
normal loopback devices, not low-latency.

And such kcontrol should not be added twice.

Change-Id: Iaab631019b5b964473bfaebade0697eb7c4373e7
Signed-off-by: Guodong Hu <guodhu@codeaurora.org>