ALSA: usb-audio: Fix packet size calculation regression

The commit d215f63d ("ALSA: usb-audio: Check available frames for
the next packet size") introduced the available frame size check, but
the conversion forgot to initialize the temporary variable properly,
and it resulted in a bogus calculation.  This patch fixes it.

Fixes: d215f63d ("ALSA: usb-audio: Check available frames for the next packet size")
Reported-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20211001104417.14291-1-colin.king@canonical.com
Link: https://lore.kernel.org/r/20211001105425.16191-1-tiwai@suse.de


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