snd-firewire-lib: add comments and arrangement for noinfo/nodata

According to IEC 61883-6:2005, there are two modes for AMDTP stream, one is
"non-blocking" (see 7.4 Transmission control) and another is "blocking" (see
Annex A (informative) Blocking transmission method).

The largest difference is the number of data block. In "non-blocking" mode,
it's differ in each packets but in "blocking" mode, it's fixed. Then, to keep
sampling rate, "no-data" packet appears in "blocking" mode.

"no-data" packet is defined in IEC 61883-1:2008 (see 6.2.2 Isochronous packet
transmission) and empty packet shall be sent. But in IEC 61883-6:2005 Annex A,
additional "special non-empty packet" is available. This packet 'has the
"NO-DATA" code in its FDF' (in the same way as nodata packet in
IEC 61883-1:2008) and 'has the same syze of dummy data as a non-empty packet'.

For convinience, this module don't send the "special non-empty" packet.

For your information, there is "no infomation" packet. This packet includes no
events which indicates syt. So all of events in this packet is within the same
syt interval. The previous and next packet includes events which indicate syt.