staging: comedi: das08: Unmark some __maybe_unused functions

The `das08jr_di_rbits()`, `das08jr_do_wbits()`, `das08jr_ao_winsn()` and
`das08ao_ao_winsn()` static functions are currently marked as
`__maybe_unused` as they were formerly only referred to by possibly
conditionally compiled out code.  This is no longer the case (they are
referred to by `das08_common_attach()`) so their `__maybe_unused` tags
can be removed.

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