gianfar: remove definition of DEBUG

Defining DEBUG should only be done in development.
So remove DEBUG.

Signed-off-by: Tom Rix <trix@redhat.com>
Link: https://lore.kernel.org/r/20210113215603.1721958-1-trix@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>