USB: remove use of the bus rwsem, as it doesn't really protect anything.

The driver core stopped using the rwsem a long time ago, yet the USB
core still grabbed the lock, thinking it protected something.  This
patch removes that useless use.

Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Oliver Neukum <oneukum@suse.de>
Cc: David Brownell <david-b@pacbell.net>
Cc: linux-usb-devel <linux-usb-devel@lists.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>