[ALSA] prevent oops & dead keyboard on usb unplugging while the device is being used

USB generic driver,USB USX2Y
Without this patch, some usb kobjects, which are parents to
the usx2y's kobjects can be freed before the usx2y's.
This led to an oops in get_kobj_path_length() and a dead
keyboard, when the usx2y's kobjects were freed.
The patch ensures the correct sequence.
Tested ok on kernel 2.6.12-rc2.

Signed-off-by: Karsten Wiese <annabellesgarden@yahoo.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>