ALSA: usx2y: Drop no longer used variable

ALSA: usx2y: Drop no longer used variable

The recent conversion to the standard print API included some cleanups
and that changed the code no longer referring to a variable iters at
usb_stream_start().  This caused a compiler warning in the end.
Let's drop the unused variable.

Fixes: f8466d91f36d ("ALSA: usx2y: Use standard print API")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202408090249.8LE9qrae-lkp@intel.com/
Link: https://patch.msgid.link/20240809075700.7320-1-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>