bebob: arrange struct snd_bebob_spec

To deal with device's differences, struct snd_bebob_spec is used. This spec
handle:
- firmware loading
- streaming parameter discoverying
- clock changing
- digital interface switching
- metering

With this commit, M-Audio Firewire 1814/Firewire 410/Firewire Audiophile
can be available just connecting. But the former two devices are still failed
to start streaming.