staging: comedi: addi_apci_3xxx: absorb apci3xxx_di_insn_bits()

All the boards supported by this driver that have digital inputs use
the same 'di_bits' callback in the boardinfo. Absorb the function from
hwdrv_apci3xxx.c directly into the driver.

Remove the 'di_bits' callback from the boardinfo and use the function
directly to set the subdevice (*insn_bits).

None of the boards supported by this driver have a 'di_config', 'di_read',
or 'io_write' callback. Remove the unnecessary setting of the subdevice
callbacks.

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>