USB: storage: Remove obsolete "FIXME"

The fix of "FIXME: Notify the subdrivers..." doesn't actually have any
 real effect. The "FIXME" changed to simple comment.

Signed-off-by: Mikhail Zaytsev <flashed@mail.ru>
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>