staging: comedi: daqboard2000: use shorter, consistent prefix

Use a consistent prefix of `db2k_` or `DB2K_` for identifiers.  The
existing prefixes `DAQBOARD2000_` and `daqboard2000_` are a bit on the
lengthy side.

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>