ASoC: msm: Fix playback failure with tinyalsa test app

PCM playback through tinyplay test app fails as it tries
to write data in chunks of ring buffer size instead of
period size. msm_pcm_playback_copy function checks the
available bytes in the shared buffer and rejects if the
bytes received are more.
Instead of rejecting the buffer, copy bytes equal to available
size i.e. period size. The ALSA framework will take care of
updating the offsets and looping through the remaining data.

Change-Id: I983b8ebedfcbd88f70d0bfbafee1930d9b793ed5
Signed-off-by: Vidyakumar Athota <vathota@codeaurora.org>