staging: gdm724x: prevent module from being built in

There are global symbols here that conflict with the rest of the kernel,
preventing it from being built, so do not allow this module to be built
in.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Won Kang <wonkang@gctsemi.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>