ALSA: firewire-lib: code refactoring for syt offset calculation

When calculating syt offset, some states are stored in AMDTP stream
structure. This is inconvenient when reuse the calculation from
non-stream structure.

This commit applies refactoring to helper function for the calculation
so that the function doesn't touch AMDTP stream structure.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20200508043635.349339-7-o-takashi@sakamocchi.jp


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