ALSA: firewire-lib: code refactoring for cache position in tx packets

ALSA: firewire-lib: code refactoring for cache position in tx packets

When sequence replay is enabled for media clock recovery, current
implementation caches sequence descriptors from packet descriptors in tx
packets. Helper function for the purpose do not necessarily have good
readability.

This commit refactors relevant functions by renaming structure members,
function name, and function local variables.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20230107023214.29132-4-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Iwai <tiwai@suse.de>