drm/msm/dp: return an error if AUX switch parsing fails

Return an error code in order to inform the caller of the
failure. In this case, the error code should trigger the
deferral of the probe for this driver.

Change-Id: I37b7747e40f10e06b49498f0a46ded6bb44274f2
Signed-off-by: Tatenda Chipeperekwa <tatendac@codeaurora.org>