staging: comedi: addi_apci_3120: tidy up timer subdevice init

It appears that this subdevice is actually broken. The subdevice functions
all violate the comedi API and, it they did work, it appears they would
break the analog input async commands.

For now just tidy up the subdevice init.

Add the SDF_READABLE subdev_flag, this subdevice provides an (*insn_read).

Fix the subdevice 'maxdata' initialization. The timer is 24-bits.

Remove the 'len_chanlist' initialization. This member is only used by
subdevices that support async commands.

Also remove the 'range_table' initialization. The core will set it to
range_unknown which is appropriate.

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>