staging: comedi: s626: remove 'pulse_index' callback from encoder private data

There are two functions used for the 'pulse_index' callback, One is used for
the channel 0-2 encoders and the other for the channel 3-5 encoders.

Refactor the two callbacks into a single s626_pulse_index() function and use
the encoder channel number to handle the differences.

Remove the then unnecessary 'pulse_index' member and just call s626_pulse_index()
drectly.

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>