In IEC 61883-1/6, one isoc packet can transfer events up to the value of
syt interval. This comes from the interval of isoc cycle. As 1394 OHCI
controller can generate hardware IRQ per isoc packet, the interval is
calculated as 125 usec.

In IEC 61883-1/6, two ways of transmission is described; blocking and
non-blocking methods. In blocking method, the sequence of packet includes
'empty' or 'NODATA' packets which include no events. In non-blocking
method, the number of events per packet is variable up to the syt
interval.

This commit uses double of the value of syt interval as minimum available
size of PCM period due to the above protocol design.
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20191017155424.885-13-o-takashi@sakamocchi.jpSigned-off-by: Takashi Iwai <tiwai@suse.de>