staging: gdm724x: Use NLMSG_HDRLEN

Replace use of  NLMSG_SPACE(0) with NLMSG_HDRLEN as they are equivalent
and NLMSG_SPACE seems to be deprecated.

Signed-off-by: Himangi Saraogi <himangi774@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>