pcm: fix underrun detection

The changes made for mmap mode broke the underrun detection by
pushing the stop_threshold beyong the buffer size.

This caused the hw_ptr to go past the app_ptr and
pcm_get_htimestamp() to report wrong values in case of underrun.

Change-Id: Ic671fdd09f3afb8a301a391b48788fd9bc99322d
1 file changed