staging/easycap: use USB_SUBCLASS_AUDIOSTREAMING instead of 0x02

use USB_SUBCLASS_AUDIOSTREAMING constant from usb/audio.h
instead of 0x02

Cc: Mike Thomas <rmthomas@sciolus.org>
Cc: Dan Carpenter <error27@gmail.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>