staging: comedi: mite: use prefered form for passing a struct size

Add a local variable to mite_buf_change() so that the prefered
form of passing a struct size, sizeof(*p), can be used.

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>