staging: dgnc: return error code directly

In various functions a return code variable is defined at the top of
function, for example;

rc = -ENODEV;

and then the variable is returned. This makes it harder to read since
it separates the error code from the return site.

Return the error code directly instead of using a variable.

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