staging: comedi: vmk80xx: fix compiler warning

gcc complains about some potentially uninitalized variables here, yet it
can not happen, due to an enumerated type (either the board is one type
or the other.)  Make the compiler happy by providing a default case
option that makes the logic a bit simpler for it to determine that there
really isn't a problem here.

Cc: H Hartley Sweeten <hartleys@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>