oxfw: fix failure to handle stream format information in LIST subfunction

Because of 'Index' field, the position of information in LIST is
1 byte back than in SINGLE.
oxfw: add condition for PLUG SIGNAL FORMAT command

With tester's help, some devices don't have oPCR. This means that AV/C OUTPUT
PLUG SIGNAL FORMAT command is failed for the devices.
oxfw: fix start/destroy streams

With tester's help, some devices don't have oPCR. This means that CMP
between the PCR is failed for the devices. Then handling streams failed.
This commit fix this failure.
oxfw: apply common way to discover for all of devices

With tester's help, devices which snd-firewire-speakers supports can respond
LIST subfunction of 'AV/C Stream Information Format' command. This commit
remove device-specific function and use common way to discover.