ALSA: fireface: support allocate_resources operation in ff800 protocol

This commit is a part of preparation to perform allocation/release
of isochronous channels in pcm.hw_params/hw_free callbacks.

This commit implements allocate_resources callback for ff800 protocol.
As I noted in commit fc716397

 ("ALSA: fireface: add support for
packet streaming on Fireface 800"), this unit allocates isochronous
resources for tx stream voluntarily. Therefore, this commit is to
maintain isochronous rsources for rx stream. In the callback,
sampling transfer frequency is configured to the device as well.

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