Revert "ASoC: msm: qdsp6v2: do not allow multiple instances of VoIP driver"

This reverts commit 0a804758 as it is not
required. pcm_open() call itself is blocking, if driver is already opened
it wouldn't allow another instance.

Change-Id: I95d1d01a1838ed6ddaaa3b00bf86476ab900d604
Signed-off-by: Venkata Narendra Kumar Gutta <vgutta@codeaurora.org>