dgnc: Move DG_PART definition from Makefile to dgnc_driver.h

Avoid deprecated usage of EXTRA_CFLAGS by moving definition of DG_PART into dgnc_driver.h

Signed-off-by: Cass May <cass@cassm.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>