staging: comedi: s626: remove my_latch_lsw from encoder private data

This member of the encoder private data can be calculated based on
the comedi channel stored in the 'chan' member of the private data.
Do that instead and remove the unnecessary member.

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>