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

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

Add a helper function, s626_set_mode(), that uses the encoder channel number
to determine which function to call.

Remove the then unnecessary 'set_mode' member and just call s626_set_mode()
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>