sfc: Consistently test DEBUG macro, not EFX_ENABLE_DEBUG

The netif_dbg() macro is defined in <linux/netdevice.h>.  If the DEBUG
macro is defined, it logs a message at 'debug' level, otherwise it
does nothing.

In net_driver.h we define DEBUG if EFX_ENABLE_DEBUG is defined, but
this is too late for those source files that already got a
definition of netif_dbg() by including <linux/netdevice.h>

Get rid of EFX_ENABLE_DEBUG, and only define and test DEBUG.

In mtd.c, we do not use DEBUG as a condition flag but are forced to
use the DEBUG macro-function from <linux/mtd/mtd.h>.  Undefine DEBUG
before including it.

Signed-off-by: Ben Hutchings <bhutchings@solarflare.com>
diff --git a/drivers/net/ethernet/sfc/mtd.c b/drivers/net/ethernet/sfc/mtd.c
index 7f61cd3..b702862 100644
--- a/drivers/net/ethernet/sfc/mtd.c
+++ b/drivers/net/ethernet/sfc/mtd.c
@@ -10,6 +10,7 @@
 
 #include <linux/bitops.h>
 #include <linux/module.h>
+#undef DEBUG /* <linux/mtd/mtd.h> has its own use for DEBUG */
 #include <linux/mtd/mtd.h>
 #include <linux/delay.h>
 #include <linux/slab.h>