This is yet more preparation for the upcoming changes.

Extend snd_usb_endpoint_next_packet_size() to check the available
frames and return -EAGAIN if the next packet size is equal or exceeds
the given size.  This will be needed for avoiding XRUN during the low
latency operation.

As of this patch, avail=0 is passed, i.e. the check is skipped and no
behavior change.

Link: https://lore.kernel.org/r/20210929080844.11583-7-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>