aplay: pcm_readv(): return read samples instead of requested upon abort

aplay: pcm_readv(): return read samples instead of requested upon abort

This patch changes the logic of pcm_readv() when abort signal has been
detected. During such condition we should return the amount of frames
actually read instead of the size requested by caller.

Currently functions pcm_read() and pcm_readv() when aborted (in_aborting
flag set) return the amount of requested frames instead of those actually
read prior to interrupt. The consequence of this is repetition of recent X
frames where X stands for amount of frames in one period. This problem is
barely visible or rather audible when the period is small like few
milliseconds because repetition of 1 [ms] of data is not-noticeable
however if we use buffer and period sizes in seconds then the problem
becomes apparent.

Example issue -> thesofproject/sof#3189

Signed-off-by: Marcin Rajwa <marcin.rajwa@linux.intel.com>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>