staging: comedi: unionxx5: allocate 'usp' before using it

As reported by the kbuild test robot, the 'usp' pointer needs to be
allocated before being used.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>