ALSA: firewire: block .remove callback of bus driver till all of ALSA character devices are released

At present, in .remove callback of bus driver just decrease reference
count of device for ALSA card instance. This delegates release of the
device to a process in which the last of ALSA character device is
released.

On the other hand, the other drivers such as for devices on PCIe are
programmed to block .remove callback of bus driver till all of ALSA
character devices are released.

For consistency of behaviour for whole drivers, this probably confuses
users. This commit takes drivers in ALSA firewire stack to imitate the
above behaviour.
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>