Staging: comedi: comedidev.h: Drop old style zero-length array

According to Documentation/Changes, the minimum gcc version required
to compile the kernel is 3.2 (this is probably outdated too).

Signed-off-by: Cheah Kok Cheong <thrust73@gmail.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>