USB: testing driver: don't free a locked mutex

Dopey thing to do and lockdep will (or should) warn.

Spotted by Daniel Walker.

Cc: Matthias Kaehlcke <matthias.kaehlcke@gmail.com>
Cc: Daniel Walker <dwalker@mvista.com>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>