staging: comedi: cb_das16_cs: tidy up analog input subdevice init

This driver currently code not support async commands. Remove the
SDF_CMD_READ subdev_flag and len_chanlist initialization from the
subdevice init.

For aesthetics, rename the (*insn_read) function.

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>