staging: comedi: addi_apci_3xxx: clarify 'b_AvailableConvertUnit'

This boardinfo defines the conversion time bases that the analog input
subdevice supports (ns, us, ms).

Add some defines to get rid of the magic numbers and clarify the code.

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>