ASoc: msm: update the pending buffer flags in underrun

Update the pending buffer flag on a driver underrun.
This causes a drain call to be rejected if the underrun
happens immediately after an EOS buffer has been given to the DSP
following a flush

CRs-Fixed: 463332
Change-Id: I2ba224a49e5af14ca909300afc0788cb4b21eb16
Signed-off-by: Subhash Chandra Bose Naripeddy <snariped@codeaurora.org>