staging: comedi: amplc_pci230: remove unused settling time constants

The macro constants `PCI230_DAC_SETTLE`, `PCI230_ADC_SETTLE` and
`PCI230_MUX_SETTLE` are never used.  Just remove them.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>