ASoC: msm: q6: Handle underrun in compressed driver

Check for underrun by calculating the difference between
hw_ptr and appl_ptr instead of calling snd_pcm_playback_empty

CRs-Fixed: 483441
Change-Id: I49aca4308769e197d9f403d6bd63c6a7addc81b0
Signed-off-by: Jitendra Singh Naruka <jnaruk@codeaurora.org>