pcm: fix a bug to copy silent samples aligned to 64

pcm: fix a bug to copy silent samples aligned to 64

bits for
  24 bit sample cases

A function of 'snd_pcm_area_silence()' has a fast path to copy silent data
efficiently. However, the fast path works well just for a case that target
buffer consists of data samples for which unit of data alignment is
divisors of 64 bits.

At present, the fast path handles sample data aligned to 24 bit. In this
case, the buffer can includes extra 8 bits. This has no issue for 'signed'
case because silent data is zero, however it has an issue for 'unsigned'
case.

This commit fixes the bug by skipping cases of sample data of 24 bit.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Jaroslav Kysela <perex@perex.cz>