staging: comedi: vmk80xx: cleanup digital input subdevice init

Change the type for the digital input 'di_chans' boardinfo to match
the comedi_subdevice type it is set to. For aesthetic reasons, rename
the variable also.

Remove the SDF_GROUND flag from s->subdev_flags. This flag only has
meaning for analog subdevices.

Add the missing s->range_table for the subdevice.

Rename the (*insn_read) and (*insn_bits) functions for the digital
input subdevice to make grepping easier.

For aesthetic reasons, add some whitespace to the subdevice init.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>