Add a workaround to handle AMDTP events correctly for Fireworks

In a routine to handle AMDTP, the number of PCM frames comes from the number of
events. The number of events is calculated with data block quadlets in AMDTP
header and data length in isochronous packet header.

But Echo's Fireworks always report a fixed value for data block quadlets even if
different sampling rate is applied.

With this workaround, the raw value in AMDTP header is not used and data block
quadlets is calculated with the numebr of PCM channels and MIDI channels.

But FFADO use the different solution. See #310.
http://subversion.ffado.org/ticket/310

I should investigate which is better, mine or FFADO's solution.