ALSA: usb-audio: Pre-calculate buffer byte size

There are a bunch of lines calculating the buffer size in bytes at
each time.  Keep the value in subs->buffer_bytes and use it
consistently for the code simplicity.

Link: https://lore.kernel.org/r/20210601162457.4877-3-tiwai@suse.de


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