ASoC: msm8x16: do not disable mclk during device switch.

MCLK is tied with the audio session, and if there is a device
switch during the sessoin mclk should not be disabled.
Update machine driver MCLK event to handle this scenario.

CRs-Fixed: 664913
Change-Id: I9155a63cd7c31160215b23a8112b941f845967ac
Signed-off-by: Santosh Mardi <gsantosh@codeaurora.org>