net: wan: wanxl: define CROSS_COMPILE_M68K

It was used but never set.  The hardcoded value from before the dawn of
time was non-standard; the usual name for cross-tools is $TRIPLET-$TOOL
Signed-off-by: NAdam Borowski <kilobyte@angband.pl>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/wan/Makefile

@@ -34,6 +34,8 @@ obj-$(CONFIG_SLIC_DS26522)	+= slic_ds26522.o
 clean-files := wanxlfw.inc
 $(obj)/wanxl.o:	$(obj)/wanxlfw.inc
 
+CROSS_COMPILE_M68K = m68k-linux-gnu-
+
 ifeq ($(CONFIG_WANXL_BUILD_FIRMWARE),y)
 ifeq ($(ARCH),m68k)
   M68KCC = $(CC)