ALSA: fireface: share helper function to get current sampling rate and clock source

As long as investigating packet dumps from Fireface 400/800, bits on
status registers for clock synchronization are the same.

This commit moves a parser for a register of clock configuration to
obsolete model-specific operations.

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