staging: comedi: daqboard2000: use designated initializers

Replace the undesignated initializers for each element of
`db2k_boardtypes[]` with an equivalent designated initializer for ease
of future maintenance.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>