ASoC: msm: qdsp6v2: Do not reset backend pass-through mode

Backend pass-through mode is reset to LEGACY_PCM during routing close.
This results in opening of adm with non pass-through path type for
pass-through session if device switch is done during pass-through
session.
Fix this by avoid resetting Backend passthrough-mode.

Change-Id: Ia1ea10ce68ba04a72416827661b6fdc2abd48b3c
Signed-off-by: Manish Dewangan <manish@codeaurora.org>