staging: comedi: vmk80xx: cleanup digital input subdevice init

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_write), (*insn_bits), and (*insn_read) 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>