ALSA: firewire-lib: code refactoring for sequence descriptor'

A internal structure is used to gather parameters relevant to sequence
descriptor.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20210518130048.146596-4-o-takashi@sakamocchi.jp


Signed-off-by: Takashi Iwai <tiwai@suse.de>