ALSA: fireface: code refactoring to handle multiplier mode

Fireface 400/800 use three modes against the number of data channels in
data block for both tx/rx packets.

This commit adds refactoring for it. Some enumerators are added to
represent each of mode and a function is added to calculate the mode
from sampling frequency code (sfc).

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>