usb-storage: change remaining semaphore to completion

This patch (as1090) converts the one remaining semaphore in
usb-storage into a completion.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Cc: Matthew Dharm <mdharm-usb@one-eyed-alien.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>