USB: mos7840: fix race in register handling

Fix race in mos7840_get_reg which unconditionally manipulated the
control urb (which may already be in use) by adding a control-urb busy
flag.

Cc: stable@vger.kernel.org
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>