ALSA: fireface: postpone sound card registration

Just after appearing on IEEE 1394 bus, this unit generates several bus
resets. This is due to loading firmware from on-board flash memory and
initialize hardware. It's better to postpone sound card registration.

This commit schedules workqueue to process actual probe processing
2 seconds after the last bus-reset. The card instance is kept at unit
probe callback and released at card free callback. Therefore, when the
actual probe processing fails, the memory block is wasted. This is due to
simplify driver implementation.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>