staging: comedi: mite: cleanup mite_unsetup()

This function is only called by comedi drivers during the (*detach) of the
driver. After it is called mite_free() is always called to kfree the allocated
mite_struct pointer.

Rename this function to mite_detach() and merge the kfree() from mite_free()
into it.

Remove the unnecessary clearing of the mite variables since the mite pointer
is going to be kfree()'d anyway.

The mite_detach() function checks if the pointer is valid so remove the
unnecessary checks in the callers.

The check can also be removed in ni_660x since the ni_660x_free_mite_rings()
function does not actually need the 'mite' pointer.

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