ALSA: firewire-lib: code refactoring for calculation of context payload

It's convenient to calculate the size of context payload apart from
the size of isochronous packet payload.

This commit adds a helper function for it.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20210520040154.80450-3-o-takashi@sakamocchi.jp


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