staging: dgnc: remove double underscore

Pre-processor header guards use double underscore, typically kernel
code uses single underscore when defining header guards. 'endif'
statement should include what is ending as a comment string.

Remove double underscore, add comment string to 'endif'

Signed-off-by: Tobin C. Harding <me@tobin.cc>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>