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>