ALSA: firewire-speakers: remove not-reused member from structure

"pcm" member in struct fwspk is used to set pcm operations but is not used
again. This commit remove this member and set pcm operations with
snd_pcm_set_ops().

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Acked-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>