staging: comedi: ni_65xx: use mite_alloc()

Allocate `struct mite_device` dynamically instead of searching for
one on the `mite_devices` list constructed by the "mite" module.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>