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

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

There are two functions used for the 'set_enable' callback, s626_set_enable_a()
function is used for the channel 0-2 encoders and s626_set_enable_b() is used
for the channel 3-5 encoders.

Refactor the two callbacks into a single s626_set_enable() function and use the
encoder channel number to handle the differenced.

Remove the then unnecessary 'set_enable' member and just call s626_set_enable()
directly.

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>